Problem: if a and b are positive integers , and a = 5 b + 20 , the greatest common divisor of a and b can not be
Rationale: 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 can not be divisive by 6 . answer : d .
annotated_formula: add(divide(20, 5), const_2)