9514 1404 393
Answer:
girls : boys = 13 : 7
7 boys in the math class
Explanation:
We assume that the number of boys is the total number of students less the number of girls. In ratio units, that is ...
girls : boys = girls : (total - girls) = 13 : (20 -13)
girls : boys = 13 : 7
There are 7 boys in the math class.
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Additional comment
We are only told the ratio, not any actual numbers. In order to answer the question, we must assume a total for the number of students in the class. It is convenient to assume that total is 20. There is nothing in the problem statement to indicate it is not some multiple of 20. The number of boys will be 7/20 of the total number of students in the math class.