一题不懂，救救！

Xiaotongdi

Three printing presses, R, 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 R, working alone at its constant rate, to do the same job?

A.8

B.10

C.12

D.15

E.20

不懂！用什么公式么？答案是E！

sdcar2010

category R     S      T
hours      r      s       t  (individually to finish the same job)

1/r + 1/s + 1/t = 1/4
1/s + 1/t = 1/5

So 1/r =1/4 - 1/5 = 1/20

r = 20