<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def time_to_first_byte(self):
 """ The aggregate time to first byte for all pages. """

        ttfb = []
        for page in self.pages:
            if page.time_to_first_byte is not None:
                ttfb.append(page.time_to_first_byte)
        return round(mean(ttfb), self.decimal_precision)