Answer: -6 °F/hr = -6°F per hour.
Explanation:
A rate of change can be defined as:
R = (total change in some quantity)/(time in which the change happened)
In this case, we know that the temperature dropped -30°F in a given time, then we already know that:
total change in some quantity = -30°F
Now, we know that this change took place between the 1:00 p.M.. and the 7:00 p.M.
The total amount of time between these two hours is:
T = (7.00hr - 1.00hr) = 6.00hr
This means that:
time in which the change happened = 6 hours.
Then we can replace these values in the above equation to get:
R = (-30°F)/(6hr) = -6 °F/hr
This means that for each hour that passes, the temperature decreases 6°F
We also can write this rate as:
R = -6 °F/hr = -6°F per hour.