Tony H. answered • 06/28/18

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Tracey's answer is absolutely correct. Just as another way to look at it though:

The three workers are all working together - The below equation is a way to quickly determine how long it takes when they work "in parallel":

1/20 + 1/25 + 1/100 = 1/T

Solve for T, and you get T=10. The time is 10 minutes.