The answer is around 137 girls. This is how you find the percentage of a number in the future.
First you change the percentage into a decimal. You do that by multipying by 100/ moving the decimal to the left twice, since multipying by 100 moves the decimal to the right twice.

Then, you multiply .27 by 508 in order to get the number of girls out of the total 508, which is ABOUT 137, since you intially get 137.16, but the questions asks for an estimate.

There you go. :)