Question

A water tank is $\frac{2}{5}$th full. Pipe A can fill a tank in 10 minutes and pipe B can empty it in 6 minutes.
What is the time taken by the tank to completely fill / empty the tank, if both the pipes are open?

• A. 6 min. to empty
• B. 6 min. to fill
• C. 9 min. to empty
• D. 9 min. to fill
• E. None of these

Answer 1

The correct option is A

6 min. to empty

Clearly, pipe B is faster than pipe A.

So, the tank will be emptied.

Part to be emptied = $\frac{2}{5}$

Part emptied by (A + B) in 1 minute = $(\frac{1}{6}-\frac{1}{10})$ = $\frac{1}{15}$

$\therefore \frac{1}{15}:\frac{2}{5}::1:x\to x=(\frac{2}{5}\times 1\times 15)=6min$

So, the tank will be emptied in 6 min.