Thanks for your help. I still cannot understand this part. Why need to find the differences of 2/10 as the question is asking how long it will take to fill up 3/10 of the tank? Pls help me to understand. Thks.

At first -> 1/2 (or 5/10) filled
In the end -> 3/10 filled
This means that water in the tank needs to be reduced by 2/10 (i.e. 14L).

Reason:
If both taps are turned on at the same time, Tap B flows at a faster rate(10.5L/min) than Tap A (7L/min).
In 1 min, Tap B will drain more water (3.5L more) from the tank than Tap A can fill.

To achieve 3/10-filled, 14L of water need to be drained from tank.
Hence, 14/3.5 = 4min

Thank you for the clear explanation. Now I understand already :)