Alex bought a notebook containing 96 pages, and numbered them from 1 through 192. Bob tore out 25 pages of Alex’s notebook, and added the 50 numbers he found on the pages. Could Bob have gotten 2012 as the sum? Show your logic steps on how you draw your conclusion.

Explanation:

Page 1 is labeled with 1 and 2, which sum to 1+2 = 3

Page 2 is labeled with 3 and 4 which sum to 3+4 = 7

Page 3 is labeled with 5 and 6 which sum to 5+6 = 11

and so on until we reach

Page 96 is labeled with 191 and 192, which sum to 191+192 = 383

Note how each page has an odd page number label and an even number label (odd on the front side; even on the back side). Adding any odd number to an even number will result in an odd number. We can prove it as such

x = some odd number = 2m+1, m is any integer

y = some even number = 2n, n is an integer

x+y = 2m+1+2n = 2(m+n)+1 = some other odd integer because it is in the form 2p+1 with p = m+n as an integer

This explains why the results 3,7,11,..,383 are all odd.

So we effectively have this set of values {3,7,11,...,383}. This is an arithmetic sequence with 3 as the first term and 4 as the common difference.

If we add two odd numbers together, we get an even number (proof is similar to one shown above)

odd + odd = even

But if we add in another odd number, then we'll go back to an odd result

odd + odd + odd = odd

If we have an odd number of odd numbers added up, then the result will be odd. In this case, we're adding 25 values from the set {3,7,11,...,383}. The value 25 is odd, so we have an odd number of values from  {3,7,11,...,383} being added up. Therefore, the result Bob will get will always be odd. There is no way to get a sum of 2012 because this value is even.

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