The ratio, by definition, is used to compare two quantities.
In this case, according to the information given in the exercise, you know that there are 8 girls and 12 boys in the running club. Therefore, the total number of people in the running club is:
[tex]8+12=20[/tex]There are 20 people in the running club.
By definition. a ratio can be written in these forms:
[tex]\begin{gathered} a\colon b \\ \\ \frac{a}{b} \end{gathered}[/tex]Then, knowing the explained above, you can determine that the ratio of boys to the total number of runners in the club, is:
[tex]\frac{12}{20}=\frac{6}{10}=\frac{3}{5}[/tex]Which can be written as:
[tex]3\colon5[/tex]Therefore, the answer is:
[tex]3\colon5[/tex]