Question

It takes 10 hours to fill a pool with water, and 20 hours to drain it. If the pool is empty and the drain is open, how long would it take to fill the pool?

Answer 1

It will take `20` hours to fill.

Explanation:

If it takes `10` hours to fill a fool, it means that after `1` hour, the pool would be `1/10` full.

If it takes `20` hours to empty the pool, it means that after `1` hour, the pool has dropped by `1/20`.

If the pool is filling and draining at the same time, then it will only fill by the difference in these amounts.

After `1` hour: `1/10 - 1/20` is what has filled up.

`2/20 -1/20 = 1/20`

So after `1` hour we have managed to fill `1/20` of the pool.

It will therefore take `20` hours to fill.