Design-Problem

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