A tyre has two punctures. The first puncture alone would have made the tyre flat in 9 minutes and the second alone would have done it in 6 minutes. If air leaks out at a constant rate, how long does it take both the punctures together to make it flat ?

\par One minute's work of both the punctures =  \frac{1}{9}+\frac{1}{6} = \frac{5}{18}

\par So, both punctures will make the tyre flat in \frac{18}{5} = 3\frac{3}{5} minutes