Question

If it takes a tub 20 minutes to fill up when the water is on all the way and 15 minutes to drain, how long will it take for the tub to be completely drained when the water is left all the way on?

In 1 hour the tub can fill up 3 times and the tub can drain 4 times. So will it take 45 minutes to be completely drained?

Answer 1

No. In 1 minute, 1/20 of the tub fills, while 1/15 drains. So, the net rate of filling is

1/20 - 1/15 = -1/60

So, in 60 minutes the full tub drains.

This assumes, of course, that the drain is opened when the tub is completely full.

Answer 2

You are right it takes 45 minutes

Answer 3

Anonymous you're wrong.