Three printing presses, E, S, and T, working together at their respective constant rates, can do a certain printing job in 4 hours. S and T, working together at their respective constant rates, can do the same job in 5 hours. How many hours would it take E, working alone at its constant rate, to do the same job?
Select
A) 8
B) 10
C) 12
D) 15
E) 20
=1/4-1/5=1/20   20 can do 1/20 job in 1 hour   20 can do the whole job in 20 hours=20
<result>E</result>
Three printing presses, E, S, and T, working together at their respective constant rates, can do a certain printing job in 4 hours. S and T, working together at their respective constant rates, can do the same job in 5 hours. How many hours would it take E, working alone at its constant rate, to do the same job?