To create a ratio you simply find the numbers that represent the items you are creating a ratio for.
In the glee club there are 5 boys and 10 girls. So your first number in the ratio is 5, because they asked for the number of boys first. Then you find the total number of all the students. So you add the 5 boys and 10 girls and you get 15 students. So 15 is your second number in the ratio because that is the second number they asked for.
