Suppose you want to drain a swimming pool at the end of the summer and you know the pool drains at 10 cubic feet per minute. The pool is 100 feet long and 40 feet wide. For the first 50 feet, the pool was water depth of 4 ft. For the next 30 feet the water depth is 7 feet and for the final 20 feet the water depth is 10 feet. How long will it take the pool to drain?

We need to calculate the volume of the pool.  Recall the volume is length times width times depth.

For the first 50 feet the volume is 50(40)(5) = 10,000 cubic feet.  For the next 30 feet the volume is 30(40)(7) = 8,400 cubic feet. For the last 20 feet, the volume is 20(40)(10) = 8,000 cubic feet. Therefore the total volume of the pool is 26,400 cubic feet.

The time it takes the pool to drain is 26,400/10 = 2,640 minutes.