Question 899874

Since the larger pump can do the job in five hours and the whole job together will take on 4 hours the larger pump will do 4/5 of the job which leaves 1/5 for the smaller pump

4/5+x/7=1
x/7=1/5
x=7/5
x=1.4 hours 
1 2/5 hours = 1 hour 24 minutes
5:00-1:24
4:60-1:24=3:36 pm 
Wait 2 hours and 36 minutes and turn on the smaller pump at 3:36 pm