A water tank is two-fifth full. Pipe A can fill the tank in 10 minutes and pipe B can empty it in 6 minutes. If both pipes are opened, how long will it take to empty or fill the tank completely ?

A 5

B 7

C 6

D 8

Correct Answer: Option C

Solution: \(\frac{{6 \times 10}}{{10 - 6}}\) = 15 m than \(15 \times \frac{2}{5}\) = 6 min