Answer
It involves multiple different types of variation.
Work Step by Step
There were three types of variation that we have studied so far. First, we studied direct variation, which is given by the equation:
$$ y = kx$$
Next, we studied inverse variation, which is of the form:
$$ xy = k$$
Lastly, we studied joint variation. This occurs when y varies with more than one different variable.
Combined variation occurs when more than one type of variation is present in the same problem.