Two taps can fill a cistern separately in 60 minutes mm 80 minutes respectively. To fill the empty cistern both the taps are kept open for 20 minutes and then the second tap is turned off. Find the required time to fill the cistern

Correct Answer: 45 minutes

In one minute two taps can fill 1/60+1/80=1/36 part

= 4+3/240=7/240 part

In 20 minutes both taps can fill 20×7/240=7/12

Remaining part = 1−7/12=5/12

Since the second tap is turned off after 20 minutes, 5/12 part can be filled by the first tap.

The first tap can fill part in 5/12×60=25 minutes

The required time to till the tank = 20+25=45minutes.