If x and y are positive integers such that y is a multiple of 5 and 3x + 8y = 700, then x must be a multiple of which of the following?
Choose the correct answer
A) 3.  B) 6.  C) 7.  D) 8.  E) 10.
If y is a multiple of 5, then 8y is also a multiple of 5.   3x = 700 - 8y   700 - 8y is even and it is a multiple of 5.   Thus x must also be even and a multiple of 5.   Therefore x must be a multiple of 10.   The answer is 10..
<result>E</result>
If x and y are positive integers such that y is a multiple of 5 and 3x + 8y = 700, then x must be a multiple of which of the following?