Pipe A can fill the tank in 30 minutes and pipe B can fill the same tank in 20 minutes. Both the pipes are kept open simultaneously. When should pipe B be switched off, so that the tank may fill in exactly 24 minutes?

A) 5 minutes
B) 7 minutes

C) 4 minutes
D) 3 minutes

Correct answer:
C) 4 minutes

Description for Correct answer:
A works for the entire duration of 24 minutes.

Therefore, in 24 min. pipe A will fill \( \Large \frac{24}{30}=\frac{4}{5} \)th of the tank

Remaining \( \Large \frac{1}{5} \)th of the tank has to be filled by pipe B in \( \Large \frac{1}{5} \times 20 \) = 4 minutes

So pipe B should be turned off in 4 minutes.

Please provide the error details in above question

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