问题31.一个袋子包含5个红色,6个白色和7个黑色的球。随机抽出两个球。找出两个球都为红色或黑色的概率。
解决方案:
Clearly the bag has 5 + 6 + 7 = 18 balls and since 2 balls have been drawn at random, number of outcomes in sample space = n{S} = 18C2 = 153
Let E be the event that the two balls chosen are either red or black.
Hence number of outcomes in event E = n{E} = 5C2 + 7C2 = 31
Probability of E = n{E}/n{S} = 31/153
Hence the probability of both balls being red or black is 31/153.
问题32.如果从英语字母表中随机选择一个字母,则求出该字母是:
(i)元音
解决方案:
We know there are 26 letters in English alphabet. Since one letter has been chosen from the 26 letters, number of outcomes in sample space = n{S} = 26C1 = 26
Let V be the event that the chosen letter is a vowel. Obviously there are 5 vowels in the English alphabet.
Hence the number of outcomes favorable to event V = n{V} = 5C1 = 5.
Probability of event V = n{V}/n{S} = 5/26.
Hence the probability of getting a vowel is 5/26.
(ii)辅音
解决方案:
Let C be the event that the chosen letter is a consonant. Obviously there are 21 consonants in the English alphabet.
Hence the number of outcomes favorable to event C = n{C} = 21C1 = 21.
Since n{S} = 26, Probability of event C = n{C}/n{S} = 21/26.
Hence the probability of getting a consonant is 21/26.
问题33.在彩票中,一个人从1到20中选择6个不同的数字,如果这6个数字与彩票委员会确定的6个数字匹配,那么他将中奖。赢得比赛的几率是多少?
解决方案:
Since there are 20 numbers, and 6 numbers have to be chosen, the number of outcomes in sample space = n{S} = 20C6 = 38760
Let L be the event that the 6 numbers chosen match with the already decided numbers. We know that there is only 1 set pattern to the numbers that have already been fixed by the lottery committee.
Hence number of outcomes favorable to event L = n{L} = 1.
Probability of event L = n{L}/n{S} = 1/38760.
Hence the probability of winning the lottery is 1/38760.
问题34. 20张卡的编号从1到20。随机抽取一张卡。找出卡片上的数字是:
(i)4的倍数
解决方案:
Since 1 card has been drawn from 20 given cards, number of outcomes in sample space = n{S}
=> 20C1 = 20.
Let Y be the event that the number on the card is a multiple of 4. We know there are 5 multiples of 4 from 1 to 20, (i.e., 4, 8, 12, 16, 20.)
Hence number of outcomes favorable to Y = n{Y} = 5
Probability of event Y = P{Y} = n{Y}/n{S} = 5/20 = 1/4.
Hence the probability of the number on the card being a multiple of 4 is 1/4.
(ii)不是4的倍数
解决方案:
Let Y be the event that the number on the card is a multiple of 4 and Y’ denote the event that it is not a multiple of 4.
We know there are 5 multiples of 4 from 1 to 20, (i.e., 4, 8, 12, 16, 20.)
Hence number of outcomes favorable to Y = n{Y} = 5
Since n{S} = 20, Probability of event Y = n{Y}/n{S} = 5/20 = 1/4.
Thus, P{Y’} = 1 − P{Y} = 1 − 1/4 = 3/4.
Hence the probability of the number on the card not being a multiple of 4 is 3/4.
(iii)奇数
解决方案:
Let O be the event that the number on the card is odd. We know there are 10 odd numbers from 1 to 20, (i.e., 1, 3, 5, 7, 9, 11, 13, 15, 17, 19.)
Hence number of outcomes favorable to O = n{O} = 10
Since n{S} = 20, Probability of event O = P{O} = n{O}/n{S} = 10/20 = 1/2.
Hence the probability of the number on the card being an odd number is 1/2.
(iv)大于12
解决方案:
Let G be the event that the number on the card is odd. We know there are 8 numbers greater than 12 from 1 to 20, (i.e., 13, 14, 15, 16, 17, 18, 19, 20.)
Hence number of outcomes favorable to G = n{G} = 8
Since n{S} = 20, Probability of event G = P{G} = n{G}/n{S} = 8/20 = 2/5.
Hence the probability of the number on the card being greater than 12 is 2/5.
(v)可除以5
解决方案:
Let F be the event that the number on the card is divisible by 5. We know there are 4 numbers divisible by 5 from 1 to 20, (i.e., 5, 10, 15, 20.)
Hence number of outcomes favorable to F = n{F} = 4
Since n{S} = 20, Probability of event F = P{F} = n{F}/n{S} = 4/20 = 1/5.
Hence the probability of the number on the card being divisible by 5 is 1/5.
(vi)不是6的倍数
解决方案:
Let Z be the event that the number on the card is a multiple of 6 and Z’ denote the event that it is not a multiple of 6.
We know there are 3 multiples of 6 from 1 to 20, (i.e., 6, 12, 18.)
Hence number of outcomes favorable to Z = n{Z} = 3
Since n{S} = 20, Probability of event Z = n{Z}/n{S} = 3/20
Thus, P{Z’} = 1 − P{Z} = 1 − 3/20 = 17/20.
Hence the probability of the number on the card not being a multiple of 6 is 17/20.
问题35.投掷两个骰子。找出赔率:
(i)赞成获得总和4
解决方案:
Since a pair of dice has been thrown, number of items in sample space = n{S}= 62 = 36.
Let E be the event of getting a sum of 4 on the dice and E’ be the event of not getting a sum of 4.
E = {(1,3), (2,2), (3,1)}
Hence number of outcomes favorable to event E = n{E} = 3
Probability of event E = P{E} = n{E}/n{S} = 3/36 = 1/12
Now, P{E’} = 1 − P{E} = 1 − 1/12 = 11/12.
Thus odds in favour of getting a sum of 4 = P{E}/P{E’} = 1:11.
(ii)赞成求和5
解决方案:
Let C be the event of getting a sum of 5 on the dice and E’ be the event of not getting a sum of 5.
C = {(1,4), (2,3), (3,2), (4,1)}
Hence number of outcomes favorable to event C = n{C} = 4
Since n{S} = 36, Probability of event C = P{C} = n{C}/n{S} = 4/36 = 1/9
Now, P{C’} = 1 − P{C} = 1 − 1/9 = 8/9.
Thus odds in favour of getting a sum of 5 = P{C}/P{C’} = 1:8.
(iii)反对获得6的总和?
解决方案:
Let A be the event of getting a sum of 6 on the dice and A’ be the event of not getting a sum of 6.
A = {(1,5), (2,4), (3,3), (4,2), (5,1)}
Hence number of outcomes favorable to event A = n{A} = 5
Since n{S} = 36, Probability of event A = P{A} = n{A}/n{S} = 5/36
Now, P{A’} = 1 − P{A} = 1 − 5/36 = 31/36.
Thus odds against getting a sum of 6 = P{A’}/P{A} = 31:5.
问题36.获得以下优势的几率是多少?
(i)如果该牌是从一副洗牌良好的牌组中抽出的,则是黑桃?
解决方案:
Let W be the event of getting a spade from a well shuffled deck of cards and W’ be the event of not getting a spade.
We know there are 13 spade cards in a pack of cards.
Hence number of outcomes favorable to W = n{W} = 13
Probability of getting a spade = P{W} = n{W}/n{S} = 13/52 =1/4
Thus, probability of not getting a spade = P{W’} = 1 − P{W} = 1 −1/4 = 3/4
Odds in favour of getting a spade = P{W}/P{W’} = (1/4)/(3/4) = 1:4
(ii)国王?
解决方案:
Let K be the event of getting a king from a well shuffled deck of cards and K’ be the event of not getting a king.
We know there are 4 kings in a pack of cards.
Hence number of outcomes favorable to K = n{K} = 13
Probability of getting a king = P{K} = n{K}/n{S} = 4/52 =1/13
Thus, probability of not getting a king = P{K’} = 1 − P{K} = 1 −1/13 = 12/13
Odds in favour of getting a king = P{K}/P{K’} = (1/13)/(12/13) = 1:12
问题37.一个盒子装有10个红色大理石,20个蓝色大理石和30个绿色大理石。随机绘制5个弹珠。找到以下可能性:
(i)都是蓝色的
解决方案:
Clearly the box has 10 + 20 + 30 = 60 marbles and since 5 marbles have been drawn at random, number of outcomes in sample space = n{S} = 60C5
Let B be the event that all 5 marbles are blue.
Hence number of outcomes favorable to event B = n{B} = 20C5
Probability of B = n{B}/n{S} = 20C5 / 60C5 = 34/11977
Hence the probability of all 5 marbles being blue is 34/11977.
(ii)至少有一个是绿色的
解决方案:
Let G be the event that at least one marble is drawn.
Probability of G = 1 − Probability of drawing no green marble = 1 − 30C5/60C5 = 1 − 117/4484 = 4367/4484
Hence the probability of drawing at least one green marble is 4367/4484.
问题38:一个盒子中有6个红色大理石,编号从1到6,有4个白色大理石,编号从12到15。求出绘制大理石的概率为:
(i)白色
解决方案:
Since 1 marble has been drawn from a box of 10 marbles, number of outcomes in sample space = n{S} = 10C1 = 10.
Let W be the event that the given marble is white.
Number of outcomes favorable to event W = n{W} = 4C1 = 4.
Probability of W = n{W}/n{S} = 4/10 = 2/5
Hence the probability of the marble being white is 2/5.
(ii)白色和奇数
解决方案:
Let Q be the event that the marble drawn is white and odd numbered. We know there are 2 odd numbers between 12 and 15, (i.e., 13 and 15).
Number of outcomes favorable to event Q = 2
Since n{S} = 10, Probability of Q = n{Q}/n{S} = 2/10 = 1/5
Hence the probability of the marble being white and odd numbered is 1/5.
(iii)偶数
解决方案:
Let V be the event that the marble drawn is even numbered.
V = {2, 4, 6, 12, 14}
Number of outcomes favorable to event V = 5
Since n{S} = 10, Probability of V = n{V}/n{S} = 5/10 = 1/2
Hence the probability of the marble being even numbered is 1/2.
(iv)红色或偶数
解决方案:
Let G1 be the event of getting a red marble. We know there are 6 red marbles, hence probability of G1 = P{G1} = 6/10
Let G2 be the event of getting an even numbered marble. We know there are 5 even numbered marbles, hence probability of G2 = P{G2} = 5/10
Now, {G1 ∩ G2} = even numbered red marble = {2, 4, 6}
Thus, n{G1 ∩ G2} = 3 and Probability of {G1 ∩ G2} = P{G1 ∩ G2} = 3/10.
Applying the law of addition,
P{G1 ∪ G2} = P{G1} + P{G2} − P{G1 ∩ G2} = 6/10 + 5/10 − 3/10 = 8/10 = 4/5
Hence the probability of getting a red or even numbered marble is 4/5.
问题39.一个班级由10个男孩和8个女孩组成。随机选择三个学生。所选组具有以下几率的可能性:
(i)所有男孩
解决方案:
Since 3 students have been selected at random from a group of 18 students, number of outcomes in sample space = n{S} = 18C3 = 816
Let B be the event that the selected group has all boys.
Hence number of outcomes favorable to B = n{B} =10C3 = 120
Probability of event B = n{B}/n{S} = 120/816 = 5/34
Hence the probability that the group has all boys is 5/34.
(ii)所有女孩
解决方案:
Let G be the event that the selected group has all girls.
Hence number of outcomes favorable to G = n{G} = 8C3 = 56
Since n{S} = 816, Probability of event G = n{G}/n{S} = 56/816 = 7/102
Hence the probability that the group has all girls is 7/102.
(iii)1个男孩和2个女孩
解决方案:
Let H be the event that the selected group has 1 boy and 2 girls.
Hence number of outcomes favorable to H = n{H} = 10C1 × 8C2 = 280
Since n{S} = 816, Probability of event H = n{H}/n{S} = 280/816 = 35/102
Hence the probability that the group has 1boy and 2 girls is 35/102.
(iv)至少1个女孩
解决方案:
Let C be the event that the selected group has at least 1 girl.
Probability of having at least 1 girl
=> 1 − P{no girl in the group} = 1 − P{all boys} = 1 − 10C3/18C3 = 1 − 5/34 = 29/34
Hence probability of having at least 1 girl is 29/34.
(v)最多1个女孩
解决方案:
Let T be the event that the selected group has at most 1 girl.
T = {0 girl, 1 girl}
Thus, n{T} = 8C0 × 10C3 + 8C1 × 10C2 = 480
Since n{S} = 816, Probability of event T = n{T}/n{S} = 480/816 =10/17
Hence probability of having at most 1 girl is 10/17.
问题40.从一张经过精心组合的52张纸牌中抽出5张纸牌。找出所有5张牌都是红心的概率。
解决方案:
Since 5 cards have been drawn from a well shuffled pack of 52 cards, number of outcomes in sample space = n{S} = 52C5 = 2598960.
Let H be the event that all 5 cards drawn are hearts. We know there are 13 hearts in a pack of 52 cards.
Hence number of outcome favorable to event H = n{H} = 13C5 = 1287.
Probability of event H = P{H} = n{H}/n{S} = 1287/2598960 = 33/66640
Hence the probability of getting 5 hearts is 33/66640.
问题41.一个袋子里装着1至20的票。找到以下可能性:
(i)两张票上都有素数
解决方案:
Since 2 tickets have been drawn from a bag containing 20 tickets, number of outcomes in sample space = n{S} = 20C2 = 190.
Let N be the event that both the tickets have prime numbers on them. We know there are 8 prime numbers between 1 to 20, (i.e., 2, 3, 5, 7, 9, 11, 13, 17, 19).
Hence number of outcomes favorable to event N = 8C2 = 28
Probability of event N = P{N} = n{N}/n{S} = 28/190 = 14/95
Hence the probability of both the tickets having prime numbers on them is 14/95.
(ii)一个有一个质数,另一个有4的倍数
解决方案:
Let T be the event that on one ticket there is a prime number and on the other there is a multiple of 4.
We know there are 8 prime numbers 5 multiples of 4 from 1 to 20.
Hence number of outcomes in event T = n{T} = 8C1 × 5C1 = 8 × 5 = 40.
Since n{S} = 190, Probability of event T = P{T} = 40/190 = 4/19
Thus the probability of getting a prime number on one ticket and a multiple of 4 on the other is 4/19.
问题42.包含7个白色,5个黑色和3个红色的球。随机抽出两个球。找到以下可能性:
(i)两个球都是红色的
解决方案:
Clearly the urn has 7 + 5 + 3 = 15 balls and since 2 balls have been drawn at random, number of outcomes in sample space = n{S} = 15C2 =105.
Let R be the event that the two balls chosen are red.
Hence number of outcomes in event R = n{R} = 3C2 = 3
Probability of R = n{R}/n{S} = 3/105 = 1/35.
Hence the probability of both balls being red is 1/35.
(ii)一个球是红色的,另一个是黑色的
解决方案:
Let X be the event that one ball is red and the other is black.
Hence number of outcomes in event X = n{X} = 3C1 × 5C1 = 15
Since n{S} = 105, Probability of X = n{X}/n{S} = 15/105 = 1/7.
Hence the probability of one ball being red and the other being black is 1/7.
(iii)一个球是白色的
解决方案:
Let D be the event that one ball is white. It means that the other ball can be from the 8 remaining balls, excluding the white balls.
Hence number of outcomes in event D = n{D} = 7C1 × 8C1 = 56
Since n{S} = 105, Probability of D = n{D}/n{S} = 56/105 = 8/15.
Hence the probability of one ball being white is 8/15.
问题43:A和B扔了一对骰子。如果A抛出9,则发现B抛出更高数字的机会。
解决方案:
Since a pair of dice has been thrown, number of items in sample space = n{S}= 62 = 36.
Let E be the event that B throws a higher number than 9, which is either 10 or 11 or 12.
Hence, E = {(4,6), (5,5), (5,6), (6,4), (6,5), (6,6)}
Hence number of outcomes favorable to E = 6
Probability of event E = n{E}/n{S} = 6/36 = 1/6
Hence the probability of B throwing a higher number is 1/6.
问题44.在惠斯德(Whist)手中,指定玩家将4个国王保住的概率是多少?
解决方案:
We know that in a hand at Whist, each given player has a pack of 13 cards.
Hence number of outcomes in sample space = n{S} = 52C13
Let K denote the event that a player has 4 kings. It implies that the player must have 9 cards from the pack of the rest of 48 cards(excluding the 4 kings already with the player.)
Number of outcomes favorable to event K = n{K} = 4C4 × 48C9 = 4 × 48C9
Probability of K = P{K} = n{K}/n{S} = 4 × 48C9 / 52C13 = 11/4165
Hence the probability that a specific player has all the 4 hearts is 11/4165.
问题45.求出一个概率,即在随机排列单词“ UNIVERSITY”的字母时,两个I在一起的可能性不大。
解决方案:
We know there are 10 letters in the word ‘UNIVERSITY’. Hence number of outcomes in sample space = n{S} = 10!
Let W denote the event that both I’s do not come together and let W’ denote the event that both I’s come together.
Hence number of outcomes favorable to event W’ = n{W’} = 2 × 9!
Thus, Probability of W’ = P{W’} = n{W’}/n{S} = 2 × 9!/10! = 2/10 = 1/5
Now, P{W} = 1 − P{W’} = 1 − 1/5 = 4/5
Hence the probability of not getting the two I’s together is 4/5.