Working alone mona can paint a room in 4 hours. lisa can paint the exact same room in 3 hours. how long should it take to paint the room if they work together?

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First find a common denominator between 4hours and 3hours. => 12 hours 
In 12 hours Mona can paint 3 rooms 
In 12 hours Lisa can paint 4 rooms 

In total they can paint 7 rooms in 12 hours. 

To paint 1 room, they need 1/7 of the time. 

12 * (1/7) = 12/7 
so 12/7 hours