If the ratio of boys to girls in a class is 3 to 4 and there are 42 pupils in the class how many are girls?

boys : girls = 3 : 4 means that there are 3x boys and 4x girls in the class. So we have:

3x + 4x = 42

7x = 42

7x/7 = 42/7

x = 6

3x = 3(6) = 18 (the number of boys)

4x = 4(6) = 24 (the number of girls)

Thus, there are 24 girls in the class.