Hi KateM, let me try to help you out with this question too.
This question makes use of a ratio concept. Most of us call it Repeated Identity.
The number of girls, after 20 percent has left, will become 4 units.
The ratio given at the end is 1 : 2.
We should manipulate the ratio for the Girls. We find the common multiple of 2 and 4 so that the Girls can “become same”.
Then we go on to work out the answer shown in my workings above.
Hope this helps! Feel free to PM me (: