RRB Junior Engineer Stage 1 Model Papers 5

Work done by A in 20 days $=\frac{80}{100}=\frac{4}{5}$
Work done by A in 1 day $\frac{4}{5\times 20}=\frac{1}{25}$....(i)
Work done by A and B in 4 days $=\frac{20}{100}=\frac{1}{5}$
(Because remaining 20% is done in 4 days by A and B).
∴ Work done by A and B in 1 day $=\frac{1}{5\times 4}=\frac{1}{20}$...(ii)
∴ Work done by B in 1 day $=\frac{1}{20}-\frac{1}{25}=\frac{1}{100}$
∴ B can complete the work in 100 days.