It is the concept of addition and subtraction that have been developed to positive value and countable blocks and objects. In addition, two groups of objects are "placed together" to see how many in total. "Subtraction says" how many or more or fewer are left "or" how many. We say that addition and subtraction are inverse operations since the other operation can be "undoed" by one operation. The inverse of 8 minus 5 is to add 3 and 5 to get 8, leaving 3.