Number Searches I

It’s been some time since there have been any puzzles to solve.  I’ve created something new (well, as far as I know) just for my blog.  I hope you enjoy it!

Many puzzle magazines have puzzles where you need to find words or numbers hidden in a grid. They may be placed in a straight line horizontally, vertically, or diagonally, and may be written either backwards or forwards. Here’s one for you — find the numbers at the right in the grid on the left.

Day031Fig1

Looks like there’s not enough room? Well, there’s a twist in this puzzle. The numbers are written in base 10 on the right, but in the grid, they’re represented in a different base, which you need to determine. Now this is a bit trickier, so I’ll give you two hints. First, the base is a prime number. And second, all the numbers in the grid are used in the completed puzzle. We’ll work out the solution next — so if you want to try to solve it on your own, now’s the time.

Impossible?  Seems like it — until you notice where the 0’s are in the grid.  Since numbers don’t begin with a 0, and since you’re given that every number in the grid must be used — that means that some numbers end in a 0, meaning they’re a multiple of the base.  And since you’re given that the base is prime, that suggests looking at the prime factorizations of the numbers on the right.

You can look at some python code to find prime factorizations of numbers — both recursive and iterative versions.

112 24 . 7
373 prime
3181 prime
8533 7 . 23 . 53
11,665 5 . 2333
14,420 22 . 5 . 7 . 103
1,354,097 29 . 53 . 881

This table shows the prime factorizations of the numbers to be found in the grid.

So what do you notice?  Since at least two of the numbers end in 0 in the grid, at least two of the numbers must share a common prime factor.  The only prime factors that any numbers have in common are 2, 5, 7, and 53.

But note that there are the digits 8 and 9 in the grid — which means that the base must be at least 10.  This eliminates all possibilities except 53.

Alternatively, you can notice that any number on the right can be expressed using at most four digits in the “mystery” base.  Since 344 is only 1,336,336, the base must be at least 35.  This would also eliminate all options except 53.

Once you know the base, the rest is fairly straightforward — once you know how to write numbers in base 53!  Luckily, I’ve also written some python code to perform this task for you, as well.  It’s the usual algorithm — find the highest power of 53 less than the number (using logarithms), find the corresponding digit, subtract, and repeat.  You’re welcome to take out your pocket calculator, write your own code, or just use mine.  But there’s no avoiding this task.

Whatever the method, you should have come up with the following results in base 53, listed in increasing order: 26, 72, 171, 320, 485, 574, and 9530.  Below you’ll see the completed puzzle.

Day031Fig2

Where did this idea come from?  It’s never possible to say exactly where an idea “comes from.”  I was thinking about taking an ordinary puzzle and adding a new twist.  The idea of using different bases popped into mind.

Then the challenge was to create a puzzle that had some interesting feature — like the 0’s.  I knew that one way to solve the puzzle would be to write a computer program which would take all the numbers in the list, and write them in different bases starting (in this case) at 35.  Then see which lists could be found in the grid.  Not so very interesting.

But with the hint and two zeroes in the grid, well, I felt I was making some progress — and I hope you’ll agree….

Now I needed to come up with another puzzle or two — which were related to the example, but which didn’t simply require going through the same steps.  Maybe only one zero instead of two?  How was that much different?  I pondered for a few minutes, and came up with the following.

In the following Number Search puzzle, find the numbers on the right in the grid on the left (written in some prime base).  Fill in all the missing numbers (which are all single digits) in the grid, and find the missing number in the list on the right (which is in increasing order).  You know that every number in the grid is used in the completed puzzle, and that the missing number on the right does not end in 0 when circled in the grid.  Good luck!

Day031Fig3

Although likely a challenge if you’ve never done a puzzle like this before, I thought it must be possible to introduce still another level of difficulty for the truly dedicated solver.  So here’s another.

In the following Number Search puzzle, find the numbers on the right in the grid on the left.  The catch?  The numbers on the right are not written in base 10 (although they are all written in the same base), and the numbers to circle in the grid are also not in base 10 (although, again, they are all in the same base).  Your task to find both bases and solve the puzzle!

Day031Fig4

The bases are prime (but I don’t think that will actually help at all).  A few other hints:  as in a usual search puzzle, you can’t count “1011” and “1101” as two different numbers — once you circle a set of numbers, you can’t use it again in the reverse order.  Of course no number in the grid begins with “0.”  And finally, all circled numbers in the grid are four digits long, and every number is used at least once.  As a further tease, you can find out the bases with just a few lines of algebra — you only need a computer/calculator to convert from one base to the other!

Next week, I’ll explain how I designed the puzzle (and in doing so, give the solution as well) — there were two big hurdles.  First, the numbers had to use only the digits 0–9 in both bases.  And second, I wanted to be sure there was some way to solve it without an exhaustive computer search.  More next week.  Happy solving!

Published by

Vince Matsko

Mathematician, educator, consultant, artist, puzzle designer, programmer, blogger, etc., etc. @cre8math

One thought on “Number Searches I”

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s