Question 186549
We can use the formula:  rate x time = 1(job done)
                         Let T be the time needed for both to paint the house
                          Sally's rate is 1/4  per hour
                          John's rate is 1/6 per hour
 So : (1/4)T +(1/6)T=1
          (3/12+2/12)T=1
           (5/12)T=1
                T=12/5
     12/5 hrs=2hours and 12 minutes.