Critical thinking?

Question

charlie doofus has a swimming pool that can be filled in 6 hours and drained in 15 hours. how long will it take to fill up the pool if doofus forgets to close the drain valve?

Answer 1

Figure out what happens in 1 hour:
In 1 hour 1/6 of the pool is filled and 1/15 of it
is drained.
So when both events happen
1/6 -1/15 of it is filled in 1 hour.
Also 1/x of it is filled in 1 hour.
So
1/6 - 1/15 = 1/x.
3/30 = 1/x
x = 30/3 = 10 hours.
It would take 10 hours to fill the pool.