Question 3221
Sally takes 4 hours to paint the house so she paints 1/4 of the house per hour.
John takes 6 hours so he paints 1/6 of the house per hour.

In one hour they paint 1/4 + 1/6 of the house.
{{{1/4 + 1/6 = 3/12 + 2/12 = (5/12) of the house}}} To paint the whole house will take 12/5 of an hour 
ie 2 hours and 24 minutes.