Question

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?

Answer 1

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 = $\frac{1}{4}$,

Time taken by S and T when they work together = 5 hours,

So, the one hour work of S and T = $\frac{1}{5}$

∵  One hour work of R= One hour work of R, S and T - one hour work of S and T

$=\frac{1}{4}-\frac{1}{5}$

$=\frac{5-4}{20}$

$=\frac{1}{20}$

Hence, the time taken ( in hours ) by R when it works alone  

$=\frac{1}{\text{One hour work}}$

$=\frac{1}{1/20}$

= 20 hours