Question 509134
Sally does 1/4 of the house in an hour and John does 1/6 of the house in an hour, so both they do 1/4 + 1/6 = 5/12 of the house in an hour.


Which means they take 12/5 hours or 2.4 hours to paint the house.


Check:
Sally’s proportion of the house is 2.4/4 = 3/5
John’s proportion of the house is 2.4/6 = 2/5
Combined they give 2/5 + 3/5 = 1 which is the full house.


You can see Sally does 3/2 times as much as John because she works 3/2 times as fast and therefore takes 2/3 of the time to do the same job (4 hours as opposed to 6 hours).


Hope this makes sense.