5 : 15
To find the ratio, we simply take the amount of boys and put that number first because that is what they asked for first:
5
Then we find the total amount of the students because that is what they ask for next:
5 boys + 10 girls = 15 students
Finally we take those two numbers and put them in a ratio:
5 : 15
(I had to redo my answer cause for some reason my other one was taken down.)