Three printing presses, \(R, S,\) and \(T,\) working together at their respective constant rates, can do a certain print

[Vincen]

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

Answer: E

Source: Official Guide