Use (1/a) + (1/b) = (1/t) formula

(1/20) + (1/30) = 5/60

60/5 = 12

Answer 12 minutes

How do you copy the practice test problem and attach it here? I've tried over and over to copy but I can not figure out how to do it.

Thanks.

Aside from Mike22629's formula to solve this problem, this problem can also be solved by doing the following:

Let the depth of the pool equal x.

The smaller hose can fill the pool at x/30 per min and the bigger hose can fill the pool at x/20 per min. From this, we can conclude that both hoses can fill the pool at x/30+x/20= x/12 per min simultaneously. Now, we need to know how many minutes it'll take the fill the pool, which is of depth x. Since both hoses fill the pool at x/12 per min, we can do (x)/(x/12) to find out how many mins it'll take them to fill the whole pool. The answer comes out to be 12.