We want to get the ratio of blue circles to the total number of circles. Based on the provided image, there are 3 blue circles and a total of 15 circles. The ratio of blue circles to the total number of circles can be written as

This can be simplified further as

Explanation as to why the attached answer is wrong:
The answer provided is correct, which is written in simplified form. Maybe try submitting the answer in the original form rather than the simplified form.
Answer: 3:15 or 3/15
Simplified form: 1:5 or 1/5