In a class of 30, the ratio of boys to girls is 2 : 3 , how many girls are there?

If the ratio is 2 : 3 then in a class of 5 there would be 2 boys and 3 girls. 

So we can say that 2/5 of the class are boys and 3/5 of the class are girls. 

And we know that to calculate 3/5 of 30 we must first divide by 5 and then multiply by 3.

30 divide by 5 is 6. 6 multiplied by 3 is 18. Therefore there are 18 girls in the class.