# Can you solve the seven planets riddle?

Originally published at: https://boingboing.net/2018/03/05/can-you-solve-the-seven-planet.html

β¦

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I think the answer is easier than that. Just go start at 6, move 2-4 and then 4-6 and you`ve got them in 2 movesβ¦ this assumes that them moving onto you means you catch them, though.

Even easier: Go to planet 3 and wait. Theyβll pass through in fewer than 10 moves.

Nothing stops the rebels from going back and forth between two outer planets the entire time.

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Easiest of all: obtain a FISA warrant and geolocate their smartphones.

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That will catch you Harry, Ron and Hermione who will then challenge you to figure out how many times theyβve seen Titanic.

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The types of states for the criminals are:

1. outer planet
2. inner planet
3. center

and possible transitions: 1 -> 2; 2 -> 1 or 2 -> 3; 3 -> 2

Note that if they start at a 1 or 3, all odd numbered turns will be at a 1 or 3 and all even ones at a 2. If they start at a 2, the opposite.

Hmm, the 2s are looking interesting. For the criminals, to change which 2 theyβre at requires going through the center.

I feel like Iβm getting somewhere, but I have to go back to work! Collaborative puzzle solving, anyone?

Good point.

What if I secretly sympathize with the rebels? Theyβre not burglars, you know, despite the way theyβre portrayed in the media.

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About that Harry Potter / Titanic riddle, Iβm too stubborn to be blackmailed into subscribing to that service so I hope someone here will tell me Iβm right, or wrong.

I think Ron watched it three times and Harry twice. The premise tells them they each watched it, so they know the other saw it, at a minimum, once.

Harry does know who saw it more, but only after he got Ronβs answer.

When Harry asks Ron if he knows who watched it more, we have to assume that Ron has put on his thinking cap and does some deduction. If Ron watched it once he would know, in that case Harry would be the one that saw it one more time, since both of them watched it at least once. If Ron watched it twice he would not know, Harry may have seen it once, or three times, the same goes for every other amount of times Ron watched it.

So because Harry knows after Ron has said he doesnβt know this must mean that Harry has seen it twice. This is the only amount of viewings where Harry can be unsure at first, and then sure after Ron let him know he is unsure.

It just stays interesting for me how this works, when I started writing my explanation I wasnβt too sure about my conclusion. In the process of writing it my answer changed a few times, now Iβm quite sure I got the right answer. Writing stuff out, making your argument clear, often makes is so much clearer for yourself too.

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Your route is cops: 6 β> 2 β> 4 β> 6 which does not intersect rebels: 2 β> 3 β> 2 β> 3 (among others).

If they start on planet 1 and move between 1 & 2 they are caught in the 6th hour:

``````Rebels: 1 β> 2 β> 1 β> 2 β> 1 β> 2
Cops:   2 β> 3 β> 4 β> 3 β> 6 β> 2
``````

If they start on planet 2 and move between 1 & 2 they are caught in the first hour

``````Rebels: 2
Cops:   2
``````

If they start on planet 5 they are caught the 8th hour:

``````Rebels: 5 β> 4 β> 5 β> 4 β> 5 β> 4 β> 5 β> 4
Cops:   2 β> 3 β> 4 β> 3 β> 6 β> 2 β> 3 β> 4
``````

If they start on planet 4 they are caught the third hour:

``````Rebels: 4 β> 5 β> 4
Cops:   2 β> 3 β> 4
``````

If they start on planet 6 they are caught the sixth hour:

``````Rebels: 6 β> 7 β> 6 β> 7 β> 6
Cops:   2 β> 3 β> 4 β> 3 β> 6
``````

If they start on planet 7 they are caught in the tenth hour:

``````Rebels: 7 β> 6 β> 7 β> 6 β> 7 β> 6 β> 7 β> 6 β> 7 β> 6
Cops:   2 β> 3 β> 4 β> 3 β> 6 β> 2 β> 3 β> 4 β> 3 β> 6
``````

Edit: And whoops! Looks like I misunderstood your point. You were simply arguing that the rebels need not pass through 3.

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But very thorough! =oD

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