initially b:g is the ratio of boys to girls with b= the number of boys and g= the number of girls

b+g = 30

g =30-b or b=30-g

b/g = b/(30-b) = (30-g)/g

if more girls enter, call the number of new girls g', then the new ratio becomes b/(g+g') =(30-g)/(g+g')

for example if b=g =15 and g'=5 the the initial ratio is 15/15 = 1/1

and the new ratio =15/20 = 3/4