I'm a little surprised this question is in the public domain already.
It's from 4MA1 (Q2, paper 1H) from June 2018.
I would have solved it totally in terms of ratio.
I wonder if the OP had considered / would consider that approach?
If you can find the ratio of
girls : boys : adults
... then the answer / working is very simple.
Question is, can you find the ratio girls : boys : adults ??
It will mean rearranging the fraction 3/5 into the form of a ratio, instead.
In my experience, most students can demonstrate this.