if hose a can fill up a swimming pool in 6 hours, hose b in 3 hours, hose c in 2 hours, how long will it take to fill up the pool using all 3 hoses?
Let V be the pool's Volume. Each hour, the hoses fill up this much of the pool:
Hose A, V/6 of the pool
Hose B, V/3 of the pool
Hose C, V/2 of...