Question

```Since there were 20% more male than female at first, it means if there
were 5u females, there would be 6u male. (6u - 5u = u is 20% of 5u)

For every 6 latecomers, 5 were females. So if there were 6p latecomers,
p were males and 5 were females. They are the darker boxes in the model.

Since total number of males and females match at the end and equal 1000,
there were 500 each. And also the combined models are equal in size.

Look at the dotted box: it says u + p = 5p. So u = 4p.
Total females at the end = 5u + 5p = 20p + 5p = 25p = 500.
So p = 20 and total males in the hall at first = 500 - p = 480.

CHECK:
At start:   male = 480,  female = 400   (so 20% more male)
Latecomers: male =  20,  female = 100   (for every 6, 5 were female)
At end:     male = 500,  female = 500

If you find the answer useful, please accept.

Btw, where is this question from? Some assessment book? Or school?
It seems like simultaneous equations in disguise, and values chosen
such that they can be solved easily without too much algebra.
P6 kids have a tough time these days. :)```