A true mathematician would know this problem. So If you know the magic square(You can look it up)I need each of a 3 by 3 square
to have one of these each number in it: 5,6,7,8,9,10,11,12, and 13. You can only use one number once.But each row,column and from diagonal must all equal up to 27.
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.