Which of these values of n makes the equation true? Substitute each one to find out.
4n + 15 = 55
n = 8 10 13 15n = 8: 4(8) + 15 = 47 ✗
n = 10: 4(10) + 15 = 55 ✓
n = 13: 4(13) + 15 = 67 ✗
n = 15: 4(15) + 15 = 75 ✗
Only n = 10 makes both sides equal, so it is the solution. Solving an equation means finding the values that make it true — substituting is a way of testing that directly.