20和451之间的多少个数字可以被9整除?
(A) 44
(B) 48
(C) 50
(D) 52答案： (B)
解释：
The required numbers are 27, 36, 45……450.
This is an A.P. with a = 27 and d = 9
Let it has n terms.
Then Tn = 450 = 27 + (n-1) x9
∴ 450 = 27+ 9n - 9
∴ 9n = 432
∴ n = 48这个问题的测验