In 400 students, the girls will be 220 since the ratio of boys to girls is 9:11. The boys will be equal to 180.
Solution:
9x + 11x = 400
20x = 400
x = 20
To get the ratio:
9 * 20 = 180 boys
11 * 20 = 220 girls
A ratio is simply a comparison between two numbers. To keep it simple, we'll ignore the units (e.g., cost in dollars or weight in ounces) and focus just on the number part for a bit. For example, how does 3 compare to 6? Well, three is half of six. We can write ratios in one of three ways:
3 : 6
3 / 6
3 to 6
Practice solving ratio word problems at Khanacademy.org.