Question 969836
Let x=time needed to paint fence when they work together
Then together they paint at the rate of 1/x fence per hour

Andrew paints at the rate of 1/7 fence per hour
Diana paints at the rate of 1/4 fence per hour
soooo
1/7 + 1/4=1/x  multiply each term by 28x to get rid of fractions
4x+7x=28
11x=28
x=28/11 hrs (you can divide this out)
CK
In (28/11) hrs, Andrew paints(28/11)*(1/7)=4/11 of the fence
In (28/11) hrs, Diana paints (28/11)*(1/4)=7/11 of the fence
Those should add up to 1 fence and they do:)
Hope this helps----ptaylor