At a rock concert, the ratio of girls to boys is 3:7. If there are 360 more boys than girls, how many girls are at the concert?

You could understand the ration as parts: the total number of parts was 3+7=10.

So girls were 3/10 parts and boys 7/10 parts. The difference was 7-3=4 parts. this difference is equal to 360, so each part is 90 (4*90 = 360).

so girls are "3 parts" and each part is 90, then the number of girls is 3*90=270.