I'm not exactly sure what you are trying to ask..
However, here is my take on your problem.
36 = Total before the day
8 = Total at the end of the day
36 - 8 = 28 This is how many packages were left.
36 = 28 + 8 This is a possible equation, it contains two terms (28, 8).
36-8=28 This is another possible equation that is equvilent to the previous suggested one.
Hope this helps!