Question

Bob and sarah need to paint a fence. bob can paint the fence in 4 hours and sarah can paint the fence in 2.5 hours. if they work together, how long will it take them to paint the fence?

Answer 1

If Bob can paint the fence in 4 hours, then in 1 hour he gets 1/4 of the fence painted.  If Sarah can paint the fence in 2.5 hours, then she gets 2/5 of the fence painted in one hour (1/2.5 = 2/5).  We add those amounts together and set them equal to 1/x, x being the the total time it will take them to paint the whole fence.  1/4 + 2/5 = 1/x.  Find a common denominator first, then add the left side together.  5/20 + 8/20 = 1/x.  13/20 = 1/x.  Cross-multiply to get 13x = 20 and x = 1.54 hours, just a bit over an hour and a half.