Painting the fence 3 solution

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Peni takes 30 hours to paint a fence. Harry takes 20 hours to paint the same fence.

How long does it take them to paint the fence together?

Solution:

Find how much of the fence each person can paint in 1 hour.

Peni can paint 1/30 of the fence per hour.
Harry can paint 1/20 of the fence per hour.

Together, they can paint 1/30 + 1/20 of the fence in 1 hour.
1/30 + 1/20 = 2/60 + 3/60 = 5/60
5/60 can be simplified to 1/12.

If they can paint 1/12 of the fence per hour it will take them 12 hours to paint the whole fence.