If an integer n is to be selected at random from 1 to 900, inclusive, what is probability n(n+1) will be divisible by 9? Choices:
A) 1 / 9
B) 1 / 3
C) 1 / 2
D) 2 / 9
E) 2 / 3

Rationale: for n ( n + 1 ) to be a multiple of 9 , either n or n + 1 has to be a multiple of 9 . thus n must be of the form 9 k or 9 k - 1 . the probability is 2 / 9 . the answer is d .
Answer: D