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 (490 =360).
so girls are "3 parts" and each part is 90, then the number of girls is 390=270.