En un colegio hay dos grupos de 1º ESO con 24 alumnos cada uno. a) Si las chicas de 1º A son el doble que los chicos, ¿cuántas chicas hay en la clase?
b) Si el número de chicas de 1º B supera en cuatro al de chicos, ¿cuántos chicos
hay?

I will answer in English.

In each group, A and B, we have 24 students.

a) The number of girls in A is twice the number of boys in A.

Then, if G is the number of girls and B the number of boys.

G + B = 24

G = 2B

Now we can replace the second equation into the first one:

2B + B = 24

3B = 24

B = 24/3 = 8

G = 24 - 8 = 16

We have 16 girls in class A.

b) In B we have 4 more girls than boys, then:

G  = B + 4

G + B = 24

we can replace the first equation into the second one:

B + (B + 4) = 24

2*B = 24 - 4 = 20

B= 20/2 = 10

We have 10 boys in class B:

picture below