Can someone explain this to me ?

Question

Can someone explain this to me

"In reading group A with 90 students, there are 4 boys for every 5 girls. In the other reading group, B, there are 3 boys for every 2 girls. If these two groups are combined, the ratio would be 10:9. How many students are in the reading group B?"

Answer 1

A has 90 with b/g =4/5

B has b/g =3/2

combine A&B ratio b/g=10/9

4(90)/5 + 3x/2 = (90+x)(10/9) where x = number in B

360/5 + 3x/2 = 100 +10x/9

72 +1.5x = 100+10x/9

3x/2 -10x/9 = 28

(27x -20x)/18 = 28

7x = 504

x = 72 in B