Explanation:

Let us assume that the tank has a capacity of 120 litres. So, the pipes discharge the following amounts of water:
(A) 4 litres per minute
(B) 2 litres per minute
(C) 1 litre per minute.
Part 1: B and C (3 litres/min) are kept open for 10 minutes, filling 3 × 10 = 30 litres.
90 litres remain to be filled in the tank.
Part 2: Now, B is shut and A is opened.
Effectively, this means that A and C are filling the tank together (5 litres / minute).
We don’t yet know how long A and C are open together.
Part 3: C is closed 10 minutes before the tank overflows.
This means that only A works for the last 10 minutes, filling 40 litres (working@4 litres/min)
Since 30 litres are filled in Part 1 and 40 litres in Part 3, the balance (50 litres) should have been filled in Part 2.
Working together, A and C fill 5 litres per minute in Part 2.
This means that they would have taken 10 minutes to fill 50 litres.
So, the entire time it took to fill the tank is:
10 + 10 + 10 = 30 mins.