This problem is in two parts. The
first part provides some building blocks which will help you to
solve the final challenge. These can be attempted in any order. Of
course, you are welcome to go straight to the Final Challenge
without looking at the building blocks!
In this problem, you will be working on a famous mathematical
puzzle called The Tower of Hanoi. There are three pegs, and on the
first peg is a stack of discs of different sizes, arranged in order
of descending size. The object of the game is to move all of the
discs to another peg. However, only one disc can be moved at a
time, and a disc cannot be placed on top of a smaller disc.
This interactivity shows the most efficient way of moving the discs
from one end to the other:
Full Screen Version
This text is usually replaced by the Flash movie.
Click on a question below to get started:
Question A
Look at the sequence below:
$1, 2, 4, 8, 16...$
Can you describe how to get from one term to the next?
Can you describe the $n^{th}$ term of the sequence?
Now try adding together terms from the sequence:
$1 + 2$
$1 + 2 + 4$
$1 + 2 + 4 + 8$
Do you notice anything interesting?
Can you predict what $1 + 2 + 4 + ... + 64 + 128$ would be?
Check to see if you are right.
How could you write the answer to $1 + 2 + 4 + ... +
2^n$?
Justify why your formula works.
Question B
What is the smallest number of moves needed to complete the
Tower of Hanoi game with:
i) One disc?
ii) Two discs?
iii) Three discs?
iv) Four discs?
Do you notice anything interesting about the way the number of
moves increases?
Can you explain any patterns you find?
Question C
The Tower of Hanoi puzzle can be completed in 3 moves with two
discs. Can you use this to work out how many moves would be needed
with three discs?
The Tower of Hanoi puzzle can be completed in 15 moves with
four discs. Can you use this to work out how many moves would be
needed with five discs?
In general, can you describe a way of working out how many
moves are needed when one extra disc is added?
Final Challenge
Explain how you could work out the number of moves needed for the
Tower of Hanoi puzzle with $n$ discs.
EXTENSION
There is a legend that a 64-disc version of the Tower of Hanoi is
being played out in a temple, and when the final move is made, the
world will come to an end. If one move is made each second, how
long would it take to complete the game with 64 discs? Do we need
to worry yet, if the first disc was moved at the very beginning of
time?