14. two pipes a and b can fill a tank in 15 minutes and 20 minutes respectively. Both the pipes are opened together but after 4 minutes, pipe a is turned off. What is the total time required to fill the tank?
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Step-by-step explanation:
A+b fill in 4 mins=4(1/15+1/20)=28/60=7/15
Remaining part to fix=1-7/15=8/15
B alone fill it (8/15)/1/20=2min40sec
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