NIFT UG 2015 Question Paper with solutions

Portion of work done by A in 5 days$=\frac{1}{20}\times 5=\frac{1}{4}$works
Remaining work $=1-\frac{1}{4}=\frac{3}{4}$works
Portion of work done by both A and B together in one day $=\frac{1}{20}+\frac{1}{10}=\frac{3}{20}$works
So,$\frac{3}{4}$ works will finished in $\frac{3}{4}\times \frac{20}{3}=5$days
Hence, whole work will be finished in 5 + 5= 10 days.