ANSWER
-10 degrees
EXPLANATION
We have that the temperature at midnight was 10 degrees.
Midnight is 12 am (12:00 am)
Over the next four hours, the temperature fell 20 degrees.
Four hours after 12 am is 4 am.
Since the temperature fell, we can find the new temperature by subtracting 20 degrees from the initial temperature.
That is:
(10 - 20) degrees
= -10 degrees
That is the temperature at 4 am.
