If a and b are positive integers, and a=20b-15, the greatest common divisor of a and b CANNOT be Choose the correct choice from the following answers:
A) 1	B) 3	C) 18	D) 15	E) 5
If b is 1, 3, 5, or 15, then GCD of a and b is 1, 3, 5, and 15 respectively. So, by POE the answer must be C.   Still: if b is a multiple of 18, then a is 15 smaller than a multiple of 18, so not a multiple of 18, so both of them cannot be divisive by 18.   Answer: 18.
<result>C</result>
If a and b are positive integers, and a=20b-15, the greatest common divisor of a and b CANNOT be