Three printing presses, W, 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 W, working alone at its constant rate, to do the same job?	Choose one.
A) 8	B) 10	C) 12	D) 15	E) 20
=1/4-1/5=1/20   W can do 1/20 job in 1 hour   W can do the whole job in 20 hours=20
<result>E</result>
Three printing presses, W, 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 W, working alone at its constant rate, to do the same job?