Final answer:
The ratio of girls to boys is 13 to 9 and with 39 girls, each 'part' in the girl's ratio is 3, which means there would be 27 boys when using this proportion. The correct answer is not listed among the given options.
Step-by-step explanation:
If the ratio of girls to boys is 13 to 9, and there are 39 girls, we can determine the number of boys using a proportion. First, we identify that there are 13 parts of girls for every 9 parts of boys. Since there are 39 girls, each 'part' in the girl's ratio represents 39 divided by 13, which equals 3 girls per part. To find the boys, we multiply the number of boys per part by 3.
Boys per part = 9
Total boys = Boys per part × Number of boys per part = 9 × 3 = 27 boys
Therefore, according to the given ratio, there would be 27 boys if there are 39 girls. This means option (c) 21 is not the correct number of boys, and the correct answer is not provided among the given options.