Mathematics > Standard Multiple Choice
Read the following SAT test question and then click on a button to select your answer.
All numbers divisible by both 4 and 15 are also divisible by which of the following?
(A) 6
(B) 8
(C) 18
(D) 24
(E) 45
The correct answer is C
Explanation
All numbers divisible by both 
and 
are the multiples of 
and 
. Since 
and 
have no prime factor in common 
, then the least common multiple of 
and 
is equal to their product, namely 
. Every other multiple of 
and 
is divisible by 
. Thus, if 
is divisible by a number then all the multiples of 
and 
are divisible by that number. Therefore, it is enough to check by which number in the given options is 
divisible. Only 
divides 
and none of the other do. The answer must be 
.
