9 1/2 (9.5) inches. If the candle burns at a rate of 1/2 an inch per hour, after 5 hours it will have burned 2 1/2 inches (5x1/2). If the original height was 12 inches, and it burned down 2 1/2 inches, you would do 12-2.5 to get a final length of 9 1/2 inches.
