If a and b are positive integers, and a=5b+20, the greatest common divisor of a and b CANNOT be	Pick one: A) 2 B) 4 C) 5 D) 6 E) 10
If b is 2, 4, 5, or 10, then GCD of a and b is 2, 4, 5, and 10 respectively. So, by POE the answer must be D.   Still: if b is a multiple of 6, then a is 20 greater than a multiple of 6, so not a multiple of 6, so both of them cannot be divisive by 6.   Answer: 6.
<result>D</result>
If a and b are positive integers, and a=5b+20, the greatest common divisor of a and b CANNOT be