Question 3221
simpliy, you are finding the average of the two when they both paint a house...
in order to find the average, you add 4 hrs with 6 hrs (which gives you 10 hrs), 
then divide by how many numbers you added, (which in this case is 2)...
10/2=5
Both of them painting a house together only takes about 5 hours according to the problem you had given me...