Choose the correct answer from the alternatives given. Two pipes A and B can fill a cistern in 20 minutes and 25 minutes respectively. Both the pipes are opened together, but at the end of 5 minutes the first is turned off. How long does it take to fill the cistern?

Using the least common multiple method, let the total capacity of the cistern be 100 units, which is the least common multiple of 20 and 25. Pipe A fills 5 units per minute and pipe B fills 4 units per minute. When both run for 5 minutes, they fill (5 plus 4) times 5, or 45 units, leaving 55 units to be filled. Pipe B must fill the remaining 55 units alone at 4 units per minute, which takes 55 divided by 4, or 13 and 3 divided by 4 minutes; the question asks for the total time after the first 5 minutes, so the correct additional time is 13 and 3 divided by 4 minutes.

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