First set up the ratio 2/5 = 10/x where x is the number of girls.
Now, we can use cross-products to find the missing value.
So we have (2)(x) = (5)(10).
Simplifying, we have 2x = 50.
Dividing both sides by 2, we find that x = 25.
So there are 25 girls in the class if there are 10 boys.
