Your younger sibling runs up to you and excitedly exclaims, "im thinking of a number. If i add to the number 2 ten times, that's is, 2+my number + my number, and so on, then the answer is 2. What is my number? You almost immediately answer zero, but are you sure? Can you find a different number(other than zero) that has been the same property? If not, can you justify that your answer is the only correct answer?