Question

A can do a work in 15 days and B in 20 days. If they together work on it for 4 days; what fraction of the work will be left?

Answer 1

A's 1 day work $=\frac{1}{15}$
B's 1 day work $=\frac{1}{20}$
(A + B)'s 1 day work $=\frac{1}{15}+\frac{1}{20}$
$=\frac{4+3}{60}=\frac{7}{60}$
(A + B)'s 4 days work $=\frac{7}{60}\times 4=\frac{7}{15}$
Remaining work $=1-\frac{7}{15}=\frac{15-7}{15}$
$=\frac{8}{15}$