Question: two pipes a and b can fill a tank in 15 minutes?
Answer: Partfilledin4minutes=4(115+120)=715Remainingpart=1−715=815PartfilledbyBin1minute=120∴120:815::1:xx=815×1×20=1023min=10min. 40sec. ∴Thetankwillbefullin=4min.