The sum of two numbers a is 57. the difference between the two numbers is 15. what are the two numbers?

Okay let's set this up as an equation
x + y = 57
y - x = 15

Let's solve this by substitution.
Change y - x = 15 to y = 15 + x.

x + (15 + x) = 57
x + 15 + x = 57
2x + 15 = 57
2x = 42
x = 21

Okay so the first number is 21, let's plug that into the first equation.

x + y = 57
21 + y = 57
y = 57 - 21
y = 36
the two numbers are 21 & 36 :)