Two pipes A and B can fill a tank in 90 minutes and 75 minutes respectively Both pipes are opened Th...........

Quantitative Aptitude ( CCS) Pipes&cistern

Two pipes A and B can fill a tank in 90 minutes and 75 minutes respectively. Both pipes are opened. The tank will be filled in just 45 minutes, if the pipe A is turned off after ?

24 min

30 min

36 min

40 min

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Explanation:

Let A be turned off after x min. then,
Part filled by (A+B) in x min. + part filled by B in (45 - x) min. = 1.
.'. x(1/90 + 1/75) + (45 -x)1/75 = 1
=> x/90 + x/75 + 45/75 - x/75 = 1
=> x/90 + 45/75 = 1
=> (5x+6*45)/450 =1
=> 5x = 450- 270
=> x = 180/5 = 36