A grade has 81 girls and 72 boys. The grade is spilt into groups that have the same ratio of boys to girls as the whole grade. How many girls are in a group that has 16 boys?

81:72 can also be written as 81/72.

If you write 18:72 as a fraction, then all you would need to do

Simplify the fraction ...

Find a common denominator which in this case is 9, 81/9:72/9 = 9/8.

Now let 'x' be the number of girls. So in order to solve this, you would create a proportion that looks something like this:

x/16 = 9/8.

Now think, 8 multiplied by what equals 16? Now we should know that 8 x 2 = 16, so all you would do is multiply 9 by 2 which results in 18.

The final answer would be 18 girls in a group of 16 boys.

Sorry if the explanation is confusing.