Ok so this is how you solve it

If pipe A fills the tank in 10 mins, means every minute it fills (1/10)th of the tank

If pipe B empties the tank in 6 mins, means every minute it empties (1/6)th of the tank

So if both the pipes are on, the net work done every minute is (1/10) - (1/6) = (6-10)/60 = -4/60 = -1/15

So now -1/15 is getting emptied every min that the two pipes are open

Consider this : Your tank is 2/5 full

Since its a fraction, we can multiply the same number in the numerator and denominator without changing the value right ?

Multiply 3 on top and below

So (2 X 3)/(5 X 3) = 6/15

So now since the part of the tank with water is 6/15 and the net work of pipes A & B is -1/15 ......

The pipes will empty the tank in 6 mins

Hence the correct answer is (a) 6 min to empty

Is your Doubt cleared ?