Productivity Rate question

[r_walid]

3 printing presses R, S and T work together at constant rates and can do a job in 4 hours.

If S and T work alone, they finish the same job in 5 hours.

How many hours can R do the job if it is to work by itself?

8
10
12
15
20

[truplayer256]

1/R+1/S+1/T=1/4

1/S+1/T=1/5

R=The time it takes R to do the job if it works alone.

1/4-1/5=1/R

1/20=1/R

R=20 Hours.

E.