Question

The ratio of the number of boys to the number of girls in a summer camp is 9 : 7.
If there are 320 children in the camp, how many boys are there?

90 boys
160 boys
180 boys
200 boys

Answer 1

180 boys

Answer 2

Here is a method that I was taught (many years ago) to solve something like this.
First say it: "9 is to 7 as number of boys is to the number of girls. Now use algebra:
Let x = number of boys then you can state
320-x = number of girls.
Now as you have already stated, 9 to 7 as the number of boys to the number of girls, you can now also state x to 320-x is the number of boys to the number girls. Now below is the way I learned to solve this: