Question 321163
Painting the house is considered as 1 job

Sally can paint a house in four hours, 
so sally can do 1/4 of the job in 1 hour
John can paint the same house in six hours.
John can do 1/6 job in 1 hour
.
They together can do 1/4 + 1/6 of the job in 1 hour.
1/4 + 1/6 = 3+2 / 12
= 5/12 of the job in 1 hour
so to do 1 job  they take 12/5 hours
= 2.4 hours
They together can paint the house in 2.4 hours