2004 AIME II Problems/Problem 15

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Problem

A long thin strip of paper is units in length, unit in width, and is divided into unit squares. The paper is folded in half repeatedly. For the first fold, the right end of the paper is folded over to coincide with and lie on top of the left end. The result is a by strip of double thickness. Next, the right end of this strip is folded over to coincide with and lie on top of the left end, resulting in a by strip of quadruple thickness. This process is repeated more times. After the last fold, the strip has become a stack of unit squares. How many of these squares lie below the square that was originally the nd square counting from the left?

Solution

Number the squares . In this case , but we will consider more generally to find an inductive solution. Call the number of squares below the square after the final fold in a strip of length .

Now, consider the strip of length . The problem asks for . We can derive some useful recurrences for as follows: Consider the first fold. Each square is now paired with the square . Now, imagine that we relabel these pairs with the indices - then the value of the pairs correspond with the values - specifically, double, and maybe (if the member of the pair that you're looking for is the top one at the final step).

So, after the first fold on the strip of length , the square is on top of the square. We can then write

(We add one because is the odd member of the pair, and it will be on top. This is more easily visually demonstrated than proven.) We can repeat this recurrence, adding one every time we pair an odd to an even (but ignoring the pairing if our current square is the smaller of the two):

We can easily calculate from a diagram. Plugging back in,

See also