You can see Seb has the least, let that be X
So Kirsty has X + 2
So Mina has X + 2 + 5
Add them up x+x+2+x+2+5 to get 3x + 9
3x +9 = 30
So 3X = 21
x = 7 so Seb has 7, Kirsty has 9 and Mina 14
If you don't like the algebraic approach you can sometimes solve these iteratively in a few steps.
Start with Mina having half of the marbles- i.e. Mina has 15 , Kirsty 10 and Seb 8: the total is 33 which is 3 too high.
So reduce each by 1 to 14, 9 and 7.
That's 30 so is the solution
I must admit I've no idea how maths/algebra is taught these days, but we were always taught that if you have the method correct you could still achieve 8/10 even if you had the wrong answer; whereas if you took an educated guess or just wrote down the answer after doing it in your head you would only attain a 5/10 even with the correct answer.
Does that still apply today?
Captain's also seems fine to me- he was just pointing xud in the right direction. Prudie's is perhaps a little easier to follow up in that the equation to solve contains only additions.
As for the working out, it's always better to include it as you can gain method marks (provided the question is worth more than one mark, and this would be worth 2 or 3) , but you would be given full marks for a correct answer even if you showed no working- unless the question says you must show your working and explain your solution.