Question

P is able to do a piece of work in 15 days and Q can do the same work in 20 days. If they can work together for 4 days, what is the fraction of work left?

Answer 1

Amount of work P can do in 1 day $=\frac{1}{15}$
Amount of work Q can do in 1 day $=\frac{1}{20}$
Amount of work P and Q can do in 1 day $=\frac{1}{15}+\frac{1}{20}=\frac{7}{60}$
Amount of work P and Q can together do in 4 days $=4\times \left(\frac{7}{60}\right)=\frac{7}{15}$
Fraction of work left $=1-\frac{7}{15}=\frac{8}{15}$