If Ben can mix 20 drinks in 5 minutes, Mary can mix 20 drinks in 10 minutes, and Harry can mix 20 drinks in 15 minutes, how much time will it take all 3 of them working together to mix the 20 drinks?
-
3 minutes and 46 seconds
-
3 minutes and 44 seconds
-
2 minutes and 46 seconds
-
2 minutes and 44 seconds
D
Correct answer
Explanation
Ben's rate: 20/5 = 4 drinks/min. Mary's rate: 20/10 = 2 drinks/min. Harry's rate: 20/15 = 4/3 drinks/min. Combined rate = 4 + 2 + 4/3 = 6 + 1.33 = 7.33 drinks/min. Time = 20 ÷ 7.33 = 2.73 minutes = 2 minutes + 0.73×60 seconds = 2 minutes 44 seconds (approximately).