Can you solve the world’s most evil wizard riddle? - Dan Finkel

View full lesson: https://ed.ted.com/lessons/can-you-solve-the-world-s-most-evil-wizard-riddle-dan-finkel

The evil wizard MoldeVort has been trying to kill you for years, and today it looks like he’s going to succeed. But your friends are on their way, and if you can survive until they arrive, they should be able to help stop him. Can you keep MoldeVort trapped long enough for help to arrive? Dan Finkel shows how.

Lesson by Dan Finkel, directed by Artrake Studio.

The evil wizard MoldeVort
has been trying to kill you for years,

and today it looks like
he’s going to succeed.

But your friends are on their way,
and if you can survive until they arrive,

they should be able to help stop him.

The evil wizard’s protective charms
ward off every spell you know,

so in an act of desperation
you throw the only object in reach at him:

Pythagoras’s cursed chessboard.

It works, but with a catch.

MoldeVort starts in one corner
of the 5x5 board.

You have a few minutes to choose
four distinct positive whole numbers.

MoldeVort gets to say one of them,
and if you can pick a square on the board

whose center is exactly
that distance away,

the curse will force him
to move to that spot.

Then he’ll have to choose
any of the four numbers,

and the process repeats until
you can’t keep him inside the board

with legal moves.

Then he’ll break free of the spell
and almost certainly kill you.

What four numbers can you choose
to keep MoldeVort trapped by your spell

long enough for help to arrive?
And what’s your strategy?

Pause the video to figure it out yourself.

Answer in 3

Answer in 2

Answer in 1

The trick here is to keep MoldeVort
where you want him.

And one way to figure out how to do that

is to play out the game
as MoldeVort would:

always trying to escape.

You’re dealing with a relatively
small board,

so the numbers can’t be too big.

Let’s start by trying 1, 2, 3, 4
to see what happens.

Moldevort could escape those numbers
in just three moves.

By saying 2, then 3,

he would force you to let him into one
of the middle points of the grid,

and then a 4 would break him free.

But that means you’ll need to allow
a number larger than 4,

which is the distance from one end
of a row to another.

How is that even possible?

Through diagonal moves.

There are, in fact, points that are
distance 5 from each other,

which we know thanks
to the Pythagorean Theorem.

That states that the squares
of the sides of a right triangle

add up to the square of its hypotenuse.

One of the most famous
Pythagorean triples is 3, 4, 5,

and that triangle is hiding all over
your chessboard.

So if MoldeVort was here, and he said 5,
you could move him to these spaces.

There’s another insight that will help.

The board is very symmetrical:
If MoldeVort is in a corner,

it doesn’t really matter
to you which corner it is.

So we can think of the corners
as being functionally the same,

and color them all blue.

Similarly, the spaces neighboring
the corners behave the same as each other,

and we’ll make them red.

Finally, the midpoints of the sides
are a third type.

So instead of having to develop a strategy

for each of the 16 spaces
on the outside of the board,

we can reduce the problem to just three.

Meanwhile, all the inside spaces
are bad for us,

because if MoldeVort ever reaches one,

he’ll be able to say any number
larger than 3 and go free.

Orange spaces are trouble too,
since any number except 1, 2, or 4

would take him to an inside space
or off the board.

So orange is out and you’ll need
to keep him on blue and red.

That means 2 is bad,

since it could take MoldeVort
to orange on the first turn.

But the four other smallest numbers,
1, 3, 4, and 5, might work.

Let’s try them and see what happens.

If MoldeVort says 1, you can make
him go from blue to red or red to blue.

And the same works if he says 3.

Thanks to our diagonals,
this is even true if he says 5.

If he says 4, you can keep him
on the color he’s already on

by moving the length of a row or column.

So these four numbers work!

Even if your friends don’t get here
right away,

you’ll be able to keep
the world’s most evil wizard

contained for as long as you need.