The number of girls in a class is 70% of the number of boys. If the class has 25 boys, how many girls are in the class?
You can set up a ratio and then cross multiply.
http://www.worldplenty.com/rat.htm#topic7
25/x = 0.3
That is, 25 boys over x (which stands for the total number of students in the class) = 30% (shown as 0.30 for multiplying purposes).
When you cross multiply (see the webpage I gave you above), you should get 83, the total number of students.
Now you know the number of boys (25) and the total number of students (83). Subtract the boys from the class total to find the number of girls in the class. What do you get?