X can copy 80 pages in 20 hours, x and y together can copy 135 pages in 27 hours. Then y can copy 20 pages in ?

$$\eqalign{ & \Rightarrow {{\text{R}}_{\text{x}}} \cr & = \frac{{80}}{{20}} \cr & {\text{ = 4 pages/hour}} \cr & \Rightarrow {{\text{R}}_{{\text{x + y}}}} \cr & = \frac{{135}}{{27}} \cr & {\text{ = 5 pages/hour}} \cr & \Rightarrow {{\text{R}}_{\text{y}}} \cr & = {{\text{R}}_{{\text{x + y}}}} - {{\text{R}}_{\text{x}}} \cr & = 5 - 4 \cr & = 1{\text{ pages/hour}} \cr & \therefore {\text{y can copy 20 pages in}} \cr & = \frac{{{\text{20p}}}}{{{\text{1p/h}}}} \cr & = {\text{ }}20{\text{ hours}} \cr} $$