posted by Nicole .
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 20 drinks.
On this one I believe I would add 5+10+15 but what do I do after that?
No, that isn't what you must do. You add the RATES of filling drinks to get the rate that they do it together. Then divide 20 drinks by that rate.
4 + 2 + (4/3) drinks/min = 22/3 drinks min, combined
Time required = 20 drinks/(22/3 drinks/min) = 60/22 = 2.73 minutes