word problem

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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?

• word problem -

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

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