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