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

Clearly,pipe B is faster than pipe A and so,the tank will be emptied.

part to be emptied = 2/5

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

$\frac{1}{15}:\frac{2}{5}::1:x$

$\frac{2}{5}*15=6mins$

so, the tank will be emptied in 6 min