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Answer: 5 milimeters.
Step-by-step explanation:
The formula for calculate the volume of a rectangular prism is:

Where "V" is the volume, "l" is the lenght, "w" is the width and "h" is the height.
You know that the volume, the lenght, and the width of this rectangular prism are:

Substitutute these values into the formula and solve for the the height "h":

Ok so you could use the equation h=t divided by w.
h represents the hours volunteered in one week.
t represents the amount of hours volunteered in the original amount of weeks.
w represents the original amount of weeks.
When you plug In the numbers you get h=72 divided by 12 which gives you h=6 so Penn volunteered 6 hours in one week.
You know the answer is reasonable because there are 168 hours in a week, so Penn could easily spend 6 hours a week volunteering.
Sorry if my explanation is confusing, but I tried to help. Good luck :)