Question

Pipes A and B can fill a cistern in 20 and 30 minutes and can empty it in 15 minutes. If
the three are opened and closed one after the other successively for 1 min each in that order,
how soon will the cistern be filled?​

Answer 1

Answer:

167 minutes

Step-by-step explanation:

Part of cistern filled in 3 minutes = 1/20 + 1/30 - 1/15 = 1/60 part.

therefore 1 - (1/20 + 1/30) = 55/60th part is filled in 3 * 55 = 165 minutes.

Remaining part of cistern to be filled = 5/60 = 1/12th part.

If A is opened, A will 1/20th part in 1 minute.

B will fill remaining portion in (1/12 - 1/20) = 2/60 = 1/30 in 1 minute.

Total time taken to fill the cistern = 165 + 1 + 1 = 167 minutes