- #1
Jabberwocky
- 8
- 0
I was hoping some of the brilliant minds here might share their thoughts on the following puzzle:
Definition. A valid clock position is exchangeable iff swapping the positions of the minute hand and the hour hand yields another valid clock position.
Example. Midnight is an exchangeable clock position.
Puzzle. How many exchangeable clock positions exist, and what are they?
Partial Solution. We've found one special case already. By symmetry we can argue that there must be either 12 more exchangeable positions (one within each hour), or there must be infinitely many. The former seems intuitively more likely.
Visually, a time of roughly 12:41 looks as though it may be exchangeable with a time near 8:03.5. And it seems like there ought to be 11 more of these for each hour.
We can represent the minute hand as [tex] e^{i\frac{2\pi}{12}m}[/tex] where [tex]m[/tex] is between 0 and 60.
And we can represent the hour hand as [tex] e^{i\frac{2\pi}{12}h}[/tex] where [tex]h[/tex] is between 0 and 12.
Then as [tex]m[/tex] goes from 0 to 60 the hour hand, [tex]h[/tex], must go from 0 to 1. So we have
[tex] h = \frac{m}{60}[/tex],
if [tex]m[/tex] is the total number of minutes passed. But by this logic I found that the only exchangeable clock position is 12:00.
I'm curious what everyone else thinks! Are there more exchangeable positions?
Definition. A valid clock position is exchangeable iff swapping the positions of the minute hand and the hour hand yields another valid clock position.
Example. Midnight is an exchangeable clock position.
Puzzle. How many exchangeable clock positions exist, and what are they?
Partial Solution. We've found one special case already. By symmetry we can argue that there must be either 12 more exchangeable positions (one within each hour), or there must be infinitely many. The former seems intuitively more likely.
Visually, a time of roughly 12:41 looks as though it may be exchangeable with a time near 8:03.5. And it seems like there ought to be 11 more of these for each hour.
We can represent the minute hand as [tex] e^{i\frac{2\pi}{12}m}[/tex] where [tex]m[/tex] is between 0 and 60.
And we can represent the hour hand as [tex] e^{i\frac{2\pi}{12}h}[/tex] where [tex]h[/tex] is between 0 and 12.
Then as [tex]m[/tex] goes from 0 to 60 the hour hand, [tex]h[/tex], must go from 0 to 1. So we have
[tex] h = \frac{m}{60}[/tex],
if [tex]m[/tex] is the total number of minutes passed. But by this logic I found that the only exchangeable clock position is 12:00.
I'm curious what everyone else thinks! Are there more exchangeable positions?