Using the given proportions, it is found that the joint relative frequency of high school students who ride the bus is of 0.28.
A proportion is a fraction of a total amount, and the measures are related using a rule of three.
The proportions relative to riding the bus in this problem are of 0.15 for middle school and 0.13 for high school, for a total of 0.28, hence, the joint relative frequency of high school students who ride the bus is of 0.28.
More can be learned about proportions at https://brainly.com/question/14398287
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