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

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