Rosanne kept a tally of the number of emails she received each day for a week. According to the table, what was the rate of change between Friday and Saturday?
Day | Emails
Tuesday | 3
Wednesday | 3
Thursday | 7
Friday | 10
Saturday | 9
Plug the numbers into the formula for rate of change and simplify.
Rate of change
 = \frac{change in value}{change in time}
 = \frac{9 emails - 10 emails}{1 day}
 = \frac{-1 emails}{1 day}
 = -1 emails per day
The rate of change between Friday and Saturday was - 1 emails per day.