The sum of two numbers is 90. Their difference is 18. Find the numbers.

A+b=90, let b>a, so b-a=18

from the first statement b=90-a, using this value of b in the second gives you:

(90-a) - a=18

90-2a=18

-2a=-72

a=36, and since b=90-a

b=90-36=54. so the numbers are

54 and 36