We first should find how 7 relates to the 21 girls.
To do this we can divide.
The 21 girls by the 7 ratio
21 divided by 7 is 3
So 1 out of 7 in the ratio is 3 girls
We can now move this over to the boys.
If 1 means 3, and their are a ratio of 5 boys
We do 5 x 3,
Which equal 15.
Their are 15 boys in 7th grade.
Hope this helps!
Brainliest is always appreciated if you feel its deserved!