Question: In a single day, it rained for many hours. From 2pm to 4pm, it rained at a rate of 4 inches per hour. From 4pm to 7pm, it rained at a rate of 3 inches per hour. And from 7pm to 9pm, it rained at a rate of 0.5 inches per hour. If the rain gauge started out the day already containing 2 inches of rainwater, how many inches of water will the gauge contain at 9pm? Think carefully first, then make a decision:
From 2pm to 4pm is 4 - 2 = 2 hours. Thus, from 2 - 4pm, raining at 4 inches per hour, it added 8 inches to the rain gauge. From 4pm to 7pm is 7 - 4 = 3 hours. Thus, from 4 - 7pm, raining at a rate of 3 inches per hour, it added 3 * 3 = 9 inches to the rain gauge. From 7pm to 9pm is 9 - 7 = 2 hours. Thus, from 7 - 9pm, raining at a rate of 0.5 inches per hour, it added 2 * 0.5 = 1 inch to the gauge. If the gauge originally contained 2 inches of water, by 9pm the gauge contained 2 + 8 + 9 + 1 = 20 inches of water. So the answer is 20.