Question

Two pipes A and B can fill a cistern in 37 minutes and 45 minutes respectively. Both pipes are opened. The cistern will be filled in just half an hour, if the B is turned off after:

A. 5 min.
B. 9 min.
C. 10 min.
D. 15 min.​

Answer 1

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We want the cistern to be filled in just 30 minutes.

Let us say Pipe A open for 30 minutes and Pipe B is open for x minutes. SO we have the relation

(30/37) + (x/45) = 1, or

(30*45) + 37x = 37*45, or

37x = 37*45 - 30*45 = 45(37/30) = 45*7, or

x = 45*7/37 = 8.514

So, the cistern gets filled in 30 minutes with Pipe A open for 30 minutes and Pipe B, open for just 8.514 minutes. Answer.

Check: (30/37)+(8.514/45) = 1.00001.

Answer 2

Answer:

9 min

30 * 6 = 180 units

remaining 225-180 = 45 units

B filled 45/5 = 9 min