The total temperature change over 5 days in pretty city was -15 degrees celsius. what was the average temperature change each day?

To find the average, we divide the total change by the number of days it took for that change to occur, so that we can know about how big the change was each day.
-15/5 = -3
So, the average temperature change each day was -3 degrees celsius

Average change = (total change) / (total time)

                       = (-15 degrees) / (5 days)

                       = (-15 / 5)  (degree / day)

                       =     -3          degree/day