There are 90 people in the cafeteria. 50 are wearing red shirts, 15 are wearing green shirts, and the rest are wearing blue shirts. what is the ratio of the people wearing blue shirts to the total number of people in the cafeteria

First we need to calculate the number of people wearing a blue shirt,
90 - 50 - 15 = 25
the ratio is actually just like division, like the way you wrote fractions. the number on top will be the number of people who wears blue, and the number at the bottom will be the total number of people
25/90
but we need to simplify that.
5/18
therefore the ratio will be 5:18