A, B and C can do a piece of work on their own in 5, 10 and 15 days respectively. Calculate the time required by B and C to work together.

Explanation

Work done by A in one day = $\frac{1}{5}$ Work done by B in one day = $\frac{1}{10}$ Work done by C in one day = $\frac{1}{15}$ Work done in one day by B and C = $\frac{1}{10} + \frac{1}{15}$ = $\frac{3+2}{30}$ = $\frac{5}{30} = \frac{1}{6}$ Time required by B and C to work together = $\frac{1}{\frac{1}{6}} = 6$ days