x = smaller number
x + 20 = larger number
x + x + 20 = 40 ← sum of two numbers is 40
2x + 20 = 40 ← combined like terms
2x = 20 ← subtracted 20 from each side
x = 10 ← divided each side by 2
x + 20 = 30 ← substituted 10 back into x + 20
10 and 30 are the two numbers
 Proof
