Which of the following numbers is a multiple of 9 ?
(A) 50
(B) 40
(C) 35
(D) 45
(E) 55
Solution 1

A number is a multiple of 9 if it is the result of multiplying 9 by an integer.

Of the answers given, only 45 results from multiplying 9 by an integer, since $45=9 \times 5$.

Solution 2

A number is a multiple of 9 if the result after dividing it by 9 is an integer.

Of the answers given, only 45 results in an integer after dividing by 9 , since $45 \div 9=5$.

ANSWER: (D)