A motor bike tyre has two punctures. The 1st puncture alone would have made the tyre flat in 27 minutes and the 2nd alone would have done it in 18 minutes. If air leaks out at a constant rate, how long does it take both the punctures together to make it flat? a) 20 4/3 minutes b) 15 4/7 minutes c) 10 4/5 minutes d) 25 4/3 minutes