Answer:
35.
Explanation:
-15 + ___ = 20
So you want to think of the numbers like they're in a line.
EX. -5, -4, -3, -2, -1, 0, -1, -2, -3, -4, -5.
All addition of positive numbers always goes to the right, all subtraction goes to the left. It's the opposite for negatives (I.E. Adding a negative is essentially subtracting a positive.)
It helps to visualize -15 and 20 on this spectrum. You need to add a number to -15 that will make it 20, so first you want to get -15 to 0. Because it's a negative, if you add 15 it's the same as subtracting -15. This makes it 0, and from there you can just add 20 to 15.
So simply, you want to convert -15 to +15 and add +15 to +20, giving you 35.