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.