Prerequisites: Addition

Subtraction deals with the taking away of numbers, like number a from number b so that: a >= b while b > a. Such a case is with:

20 - 10 = 10.

a = 20 and b = 10.

So a - b = 10, a >= b is true, and b > a is initially true.

Subtraction is the inverse of addition, meaning that it is the opposite.