Whatever you do, do not freak out when you see these problems on the asvab. I have taken this test before and it seems difficult at first but all these problems are, are just word problems. Usually you do not need to know any special formulas.
If Steven can mix 20 drinks in 5 minutes, Sue can mix 20 drinks in 10 minutes, and Jack can mix 20 drinks in 15 minutes, how much time will it take all 3 of them working together to mix the 20 drinks?
In this problem, you need to think of proportions.
I believe that you add up all of the minutes together and divide by the number of drinks.