2/5 of a water tank is full. Pipe a fills the tank in 10 minutes, pipe B empties it in 6 minutes. Both the pipes if opened together, how long shall it take to fill or empty the entire tank?
Answer: A
There are two important points to learn in this type of question,
First, if both will open then tank will be empty first.
Second most important thing is,
If we are calculating filling of tank then we will subtract as (filling-empting)
If we are calculating empting of thank then we will subtract as (empting-filling)
So lets come on the question now,
Part to emptied 2/5
Part emptied in 1 minute =
1/6−1/10
=1/15
=>1/15:2/5 :: 1:x
=>2/5∗15
= 6mins