Question 1183385

Hello

A pipe can empty a tank in 40 minutes.
A second pipe with diameter twice as much as
 that of the first is also attached with the tank to empty it.

The two pipes together can empty the tank in 8
minutes.

I don't understand how they got 8?

How would I go about solving this?

Thanks in advance,
<pre>8 minutes is GIVEN as part of the problem. You DON'T have a question that needs to be answered.
Is the time needed for the 2nd pipe to empty the tank, by itself, needed?
That's a question that's usually asked.