从 2 到 198 的所有 10 的倍数的平均值是多少?
(一) 90
(乙) 100
(三) 110
(四) 120答案：(乙)
解释：
From 2 to 198 there are 19 multiples of 10.
These are 10,20,30.....180,190.
This is an A.P series, whose sum is (n/2{a1 + aN}),
where a1 and aN are the 1st and last terms respectively.
Putting in the formula,
a1 = 10, aN = 190.
Sum = Sn = (19/2{10 + 190 }) = 1900
Average = Sum / Total number = 1900 / 19 = 100. 这个问题的测验