Question 935152
let g=girls
b=boys

the ratio is 5:3=5/3
b/g=5/3

cross mltiply 

b=g(5/3)------eq 1

b + g = 120-----eq 2

solve g and b substitution 

substitute eq 1-eq 2 

g(5/3)+g=120

getting common denominator to add directly 

(5g+3g)/3=120

cross multiply 

8g=360


then divide it by 8

8g/8=360/8

g=45--------------------girls number 

for boys 

use eq -1

b=g(5/3)
b=45(5/3)

b=75---------boys number 

checking 

student number=120

75+45=120-----