There are 2 pipes. One is mainly used to fill up pool with water and another - to drain. It takes 2 hours more to fill up pool with water, than to drain. When we filled of the pool with water, all 2 pipes were opened and the pool drained in 8 hours. How long does it take to fill up pool, when only 1 pipe is open?
I've spent long time figuring this out, but never came close to answer - 8 hours.
What the equation should look like? ;\