Two pipes a and b can fill a tank in 20 and 30 minutes respectively. both the pipes are opened together but after 5 minutes pipe b is closed. what is the total time required to fill the tank
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total time taken to fill the tank =15mins
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total time required =22.5min
on first five min 'a' fills 1/4 of tank and 'b' fills 1/6 of tank so total of 5/12 is filled in first 5 min. Balance of 7/12 is filled by pipe 'b' . . 'b' takes 2.5 min to fill 1/12th of tank. to fill 7/12 of tank, 'b' takes (7*2.5)=17.5 min. So total time required to fill the tank is (17.5+5)min = 22.5min
on first five min 'a' fills 1/4 of tank and 'b' fills 1/6 of tank so total of 5/12 is filled in first 5 min. Balance of 7/12 is filled by pipe 'b' . . 'b' takes 2.5 min to fill 1/12th of tank. to fill 7/12 of tank, 'b' takes (7*2.5)=17.5 min. So total time required to fill the tank is (17.5+5)min = 22.5min
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