Pipe "A" and "B" can fill a tank in 15 and 20 minutes respectively. How long would it take both pipes, working together to fill it? - IQ786

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Pipe "A" and "B" can fill a tank in 15 and 20 minutes respectively. How long would it take both pipes, working together to fill it?

Pipe "A" and "B" can fill a tank in 15 and 20 minutes respectively. How long would it take both pipes, working together to fill it?

11 minutes

60/7 minutes

11/4 minutes

None of these

Pipe A fills the tank in 15 minutes (rate = 115151 per minute).

Explanation

Pipe A fills the tank in 15 minutes (rate = $\frac{1}{15}$ per minute).
Pipe B fills the tank in 20 minutes (rate = $\frac{1}{20}$ per minute).
Combined rate: $\frac{1}{15} + \frac{1}{20} = \frac{7}{60}$ per minute.
Time taken together: $\frac{1}{\frac{7}{60}} = \frac{60}{7}$ minutes.