well 12 times 50 is 600
and 12 times 49 is 588 so you are rounding 49 to 50 to make it easier to solve the question
and 588 rounds to 600 so if one answer is 1 more than the answer than the answer would be 2 less than the the another number
it could also be the other way around so it could be 2 more than the the another number