find the no. of boys

yeah totally thanks 

If 5/7 are boys, than 2/7 must be girls.

Let x be the total number of students.

Then 2/7 of x = 300     or, in math terms

(2/7)x = 300        Multiply both sides by the reciprocal of (2/7) ⇒ (7/2)

x = 300(7/2) = 1050 = total students

Then.... the total students - the number of girls = the number of boys

So 1050 - 300 = 750      And that's the number of boys.

Note something else......if 2 parts of the 7 are girls = 300, then there must be 150 students in each "part"  

Then, since the boys are 5 "parts", then 5(150) = 750 !!!

Does that make sense??