AC, my mum solved this qn using Algebra.. although the ans is the same as urs, but is it correct?
I have add her ans, can u please advise.
Thank u

Sorry late reply, Singtel service interruption now.

Take note: typo (error) on my equation, it should be:
0.5u + 0.375 = 0.2u + 2.625

Both your mum and my method basically are same.
We know the number of children are equal (unchanged) = u
I am using total orange juice when all children take.
Therefore, total orange juice when each take 1/2 (or 0.5 L) = when each take 1/5 (or 0.2L).

Your mum using number of children take enough for 1/2 L. {child that take 1/8L not included}
Therefore, children take enough 1/2L - last child (1/8L) = same number of child take 1/5L + last child (1/5L) + left ( 2. 5/8L)
Hence, u = 9 compared to my u=10.

Ok.. i understand now.
Thank u so much for clearing my doubts.
Have a good night