Can you solve the demon dance party riddle? - Edwin Meyer
Once each year, thousands of logicians
descend into the desert for Learning Man,
a week-long event they attend
to share their ideas,
think through tough problems...
and mostly to party.
And at the center of that gathering
is the world’s most exclusive club,
where under the full moon,
the annual logician’s rave takes place.
The entry is guarded
by the Demon of Reason,
and the only way to get in is to solve
one of his dastardly challenges.
You’re attending with 23
of your closest logician friends,
but you got lost on the way
to the rave and arrived late.
They're already inside, so you must
face down the demon alone.
He poses you the following question:
When your friends arrived,
the demon put masks on their faces
and forbade them from communicating
in any way.
No one at any point could
see their own masks,
but they stood in a circle where they
could see everyone else’s.
The demon told the logicians that he
distributed the masks in such a way
that each person would eventually be able
to figure out their mask’s color
using logic alone.
Then, once every two minutes,
he rang a bell.
At that point, anyone who could
come to him
and tell him the color of their mask
would be admitted.
Here’s what happened:
Four logicians got in at the first bell.
Some number of logicians, all in red
masks, got in at the second bell.
Nobody got in when the third bell rang.
Logicians wearing at least two different
colors got in at the fourth bell.
All 23 of your friends played
the game perfectly logically
and eventually got inside.
Your challenge, the demon explains,
is to tell him how many people gained
entry when the fifth bell rang.
Can you get into the rave?
Pause here to figure it out yourself.
Answer in 3
Answer in 2
Answer in 1
It’s initially difficult to imagine
how anyone could,
using just logic and the colors
they see on the other masks,
deduce their own mask color.
But even before the first bell, everyone
will realize something critical.
Let’s imagine a single logician
with a silver mask.
When she looks around, she’d see
multiple colors, but no silver.
So she couldn’t ever know that silver
is an option,
making it impossible for her to logically
deduce that she must be silver.
That contradicts rule five, so there must
be at least two masks of each color.
Now, let’s think about what happens
when there are exactly two people wearing
the same color mask.
Each of them sees only one mask
of that color.
But because they already know
that it can’t be the only one,
they immediately know that their own mask
is the other.
This must be what happened
before the first bell:
two pairs of logicians each realized
their own mask colors
when they saw a unique color in the room.
What happens if there are three people
wearing the same color?
Each of them—A, B and C—
sees two people with that color.
From A’s perspective, B and C would be
expected to behave the same way
that the orange and purple pairs did,
leaving at the first bell.
When that doesn’t happen,
each of the three realizes that they are
the third person with that color,
and all three leave at the next bell.
That was what the people
with red masks did—
so there must have been three of them.
We’ve now established a basis
for inductive reasoning.
Induction is where we can solve
the simplest case,
then find a pattern that will allow
the same reasoning
to apply
to successively larger sets.
The pattern here is that everyone
will know what group they’re in
as soon as the previously sized group
has the opportunity to leave.
After the second bell,
there were 16 people.
No one left on the third bell,
so everyone then knew there weren’t
any groups of four.
Multiple groups,
which must have been of five,
left on the fourth bell.
Three groups would
leave a solitary mask wearer,
which isn’t possible,
so it must’ve been two groups.
And that leaves six logicians outside
when the fifth bell rings:
the answer to the demon’s riddle.
Nothing left to do but join your friends
and dance.
