Greetings!
You can solve this with a simple equation:
Take the letter x to be the unknown number:
20 = 8+ (X + 5)
Then you can move the 8 + to the opposite side making it a negative 8:
20-8 = X + 5
12 = X + 5
Move the +5 over to the other side, making it a negative 5:
12 - 5 = x
7 = x
So the value of x is 7.
Hope this helps!