The addition property of equality is the idea we can add some number to both sides of an equation. You must add the same number to both sides to keep things balanced.
It's asking "what number can we add to both sides of this equation so that we isolate x?"
Think of 20+x as x+20. We can rearrange terms since adding in any order doesn't matter (eg: 2+3 = 3+2 = 5)
So we really have this equation: x+20 = 25
We can add -20 to both sides to cancel out the +20 on the left side
x+20 = 25
x+20+(-20) = 25+(-20) ...... add -20 to both sides
x = 5
This is the exact same as subtracting 20 from both sides. So 5 will go where x is, meaning that 20+x = 20+5 = 25