This is a Before After question.
This question is the skill practice on the idea of Difference.
At before, there is a difference
9u – 5u = 4u
At after, there liquid are equal.
This means that the difference in transfer (used) is the same.
4u = 2/5
1u = 2/5 ÷ 4 = 1/10
5u = 1/10 × 5 = 5/10 = 1/2 litres
At Before there is a difference.
Then an equal transfer is made.
The difference at After is the same as that at Before.