There are 75 girls and 45 boys in the choir.
You can find this by adding each side of the ratio (5 + 3 = 8) and divide 120 by the sum (120/8). This would give you 15. Now you multiply each side individually by 15, to get the number of girls and boys in the choir (5*15 = 75) and (3*15 =45). To check this, you can add the 2 products to see if it equals the total (45 + 75 = 120).
I hope this helps!