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 :)