Can you solve the giant iron riddle? - Alex Gendler
The family of giants you work for
is throwing a fancy dinner party,
and they all want to look their best.
But there’s a problem –
the elder giant’s favorite
shirt is wrinkled!
To fix it, you’ll need to power up…
the Giant Iron.
The iron needs two giant
batteries to work.
You just had 4 working ones
and 4 dead ones in separate piles,
but it looks like the baby giant
mixed them all up.
You need to get the iron working
and press the giant shirt, fast –
or you’ll end up being the
main course tonight!
How can you test the batteries
so that you’re guaranteed to get
a working pair in 7 tries or less?
Pause the video now if you want to
figure it out for yourself
Answer in 3
Answer in 2
Answer in 1
You could, of course, take
all eight batteries
and begin testing the 28
possible combinations.
You might get lucky within
the first few tries.
But if you don’t, moving the giant
batteries that many times
will take way too long.
You can’t rely on luck –
you need to assume the worst
possibility and plan accordingly.
However, you don’t actually need
to test every possible combination.
Remember – there are four
good batteries in total,
meaning that any pile of six you choose
will have at least two
good batteries in it.
That doesn’t help you right away,
since testing all six batteries could
still take as many as 15 tries.
But it does give you
a clue to the solution –
dividing the batteries into smaller
subsets narrows down the possible results.
So instead of six batteries,
let’s take any three.
This group has a total of three
possible combinations.
Since both batteries have to be
working for the iron to power up,
a single failure can’t tell you whether
both batteries are dead, or just one.
But if all three combinations fail,
then you’ll know this group has either
one good battery, or none at all.
Now you can set those three aside
and repeat the process for
another three batteries.
You might get a match, but if every
combination fails again,
you’ll know this set can have
no more than one good battery.
That would leave only two
batteries untried.
Since there are four good
batteries in total
and you’ve only accounted for two so far,
both of these remaining ones must be good.
Dividing the batteries into
sets of 3, 3, and 2
is guaranteed to get a working result
in 7 tries or less,
no matter what order
you test the piles in.
With no time to spare,
the iron comes to life,
and you manage to get the shirt
flawlessly ironed.
The pleased elder and his family show up
to the party dressed to the nines
… well, almost.
