Neither, it's a view responbsibility, ideally using a helper function. Why? Because it's about formatting and presentation.
number_to_percentage() will do the job nicely.
Paul
Probably a model task. Depends on what your calculating an for what purpose. For example calculating the total value of an order across all the order line items is certainly a model responsibility. Adding up a whole bunch of numbers that the user has ticked on a form would be more in the controller's domain.
Cheers, Max
I'd put it in the model. It is clearly a business-logic-type task.
Max