Answer:
Press R will take 20 hours.
Step-by-step explanation:
Given,
The time taken by R, S and T when they work together = 4 hours,
So, the one hour work of R, S and T = ,
Time taken by S and T when they work together = 5 hours,
So, the one hour work of S and T =
∵ One hour work of R= One hour work of R, S and T - one hour work of S and T
Hence, the time taken ( in hours ) by R when it works alone
= 20 hours