Question

someone came into my shop and stole $100 out of the register. the same person came back 5 min later with the same $100 and purchased $70 dollars worth of items and i gave them back $30 dollars in change. how much money did i lose?

Answer 1

You lost $100 at the beginning so your balance is -100
Now he came in and bought $70 worth of stuff which is -100 + 70 = -30 
but you gave him -30 back so -30 - 30 = -60

You lost 60 dollars.

Answer 2

$30, because you were robbed of $100, then given back $100, then ou gave back $30 after keeping $70 of the $100. Hope that made sense. But basically, -100, +100, -30.