<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def percent(self, value: float) -> 'Size': """Set the percentage of free space to use."""

        raise_not_number(value)
        self.maximum = '{}%'.format(value)
        return self