How much is p% of 2200?

50% of 2200 is... 50/100 times 2200, or 50 times 22.

30% of 2200 is... 30/100 times 2200, or 30 times 22.

15% of 2200 is... 15/100 times 2200, or 15 times 22.

[You should have been taught this idea of percentage at some point before Primary 6, or you might have used a different, but related, approach for calculation]

p% of 2200 is... p/100 times 2200, or p times 22.

So, 22p students are boys.

The remaining (2200 - 2p) students are girls.

We do such algebraic questions in the way we would usually do for regular questions, except that the final answer (and in some cases, the intermediate workings) will contain some letter.