By posting in "Odds and Ends," you have denied us the chance to craft help that will correlate with what you are studying. If I have to guess, it looks like a beginning algebra problem. Please post in the most appropriate forum in the future.
Assuming you are studying algebra, start by writing down symbols for each numerical quantity that may become relevant. For example,
[MATH]b = \text {number of boys.}[/MATH]
[MATH]g = \text {original number of girls.}[/MATH]
[MATH]r = \text {girls remaining after midday.}[/MATH]
With three unknowns, you will need three independent and consistent equations to find them.
What are those equations?