Hello!
You don't have to worry about the time when the temperature drops.
I believe your answer is 25°F.
If it is 20°F outside, then it takes 20°F to drop the zero. Now, since we are at zero now, and we are trying to get to -5°F. Then all we have to do is add 5 plus 20 and that is 25°F.
I hope it helps!
