Pipes and Cistern
 A tap can fill a cistern in 8 hours and another can empty it in 16 hours. If both the taps are opened simultaneously, the time (in hours) to fill the tank is ?

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Part filled by intel in 1 hour = 1/8
Part emptied by outlet in 1 hour = 1/16
Net filling in 1 hour = (1/8  1/16) = 1/16Correct Option: C
Part filled by intel in 1 hour = 1/8
Part emptied by outlet in 1 hour = 1/16
Net filling in 1 hour = (1/8  1/16) = 1/16
∴ Time taken to fill the tank = 16 hours
 Pipes A and B can fill a tank in 10 hours and 15 hours respectively. Both together can fill it in ?

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Part filled by A in 1 hour = 1/10
Part filled by B in 1 hours = 1/15
Part filled by (A + B) in 1 hour = (1/10 + 1/15) = 5/30 = 1/6Correct Option: B
Part filled by A in 1 hour = 1/10
Part filled by B in 1 hours = 1/15
Part filled by (A + B) in 1 hour = (1/10 + 1/15) = 5/30 = 1/6
∴ Both pipes together can fill the tank in 6 hours.
 Three are two taps to fill a tank while a third to empty it. When the third tap is closed, they can fill the tank in 10 minutes and 12 minutes respectively. If all the three taps be opened, the tank is filled in 15 minutes. If the first two taps are closed, in what time can the third tap empty the tank when it is full ?

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Part emptied by the third pipe in 1 min (1/10 + 1/12)  1/25 = 7/60
Correct Option: C
Part emptied by the third pipe in 1 min (1/10 + 1/12)  1/25 = 7/60
So, the full tank will be emptied by third pipe in = (60/7) min. = 8 min. 34 ec.
 A tap can fill a tank in 25 minutes and another can empty it in 50 minutes. Find whether the tank will be filled up or emptied and in how many minutes ?

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T = (25 x 50) / (50  25) = + 50 minutes
Correct Option: B
T = (25 x 50) / (50  25) = + 50 minutes
+ ve sign shows that tank is filled up in 50 minutes.