Problem: if a and b are positive integers , and a = 20 b - 15 , the greatest common divisor of a and b can not be
Rationale: "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 can not be divisive by 18 . answer : c ."
annotated_formula: add(divide(15, 20), const_2)