If you multiply two integers together and
then add 4, the result is 40. Which of the
following could NOT be the sum of the
two numbers?
Pick one
A) 12  B) 13  C) 15  D) 18  E) 20

Rationale: let the two integers equal x and y , and then create the following equation and simplify : xy + 4 = 40 xy = 36 so x and y are a pair of integers that equal 36 . try adding all possible combinations of two integers that multiply out to 36 : 1 × 36 = 36 1 + 36 = 37 2 × 18 = 36 2 + 18 = 20 3 × 12 = 36 3 + 12 = 15 4 × 9 = 36 4 + 9 = 13 6 × 6 = 36 6 + 6 = 12 this list of sums includes 12 , 13 , 15 , and 20 , but not 18 . thus , no pair of integers both satisfies the original equation and adds up to 18 correct answer d ) 18
Answer: D