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