John can paint a room in 3 hours, Mark can paint a room in 12 hours. They want to paint a room together to save time. How long would it take?

Question

John can paint a room in 3 hours, Mark can paint a room in 12 hours.  They want to paint a room together to save time.  How long would it take?

owlfred · Answer

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anonymous · Answer

In 1 hr, John will paint 1/3 of room
In 1 hr, Mark will paint 1/12 of room
Together in 1 hr, they will paint 1/3 + 1/12 = 5/12 of room
So together they will finish the work in 12/5 hrs = 2 hr 24 min