Sandy Wood
Brain Game: Prime Time
by Sandy Wood - July 8, 2009 - 7:30 AM

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I try very hard to produce original content for the Brain Game, so it’s nice to have the occasional holiday. In fact, our Web editor, Jason, recently suggested that we ask readers to send in their favorite puzzles. That way, other puzzlemakers could earn exposure on mentalfloss.com, and Kara and I could take our first real break in over a year. Selfishly, we jumped at the suggestion. So if you’ve got a puzzle that you’d like to share, please send it to me at trivia@mentalfloss.com. If your submission makes the cut, I’ll give you full credit and will also send you the T-shirt of your choice from the mental_floss store. (That’s how badly I need a vacation.)

Students should know this, but here’s a quick refresher for those of us who don’t: A prime number is one that is evenly divisble only by itself and 1. With that knowledge in mind, try your hand at today’s Brain Game:

What is the largest three-digit prime number?

Here is the ANSWER.
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Comments (10)

  1. Okay, that one may have been too easy :)

    posted by Mike's Girl on 7-8-2009 at 8:50 am

  2. Oh my gosh – I *finally* got one!

    posted by MarkT on 7-8-2009 at 8:55 am

  3. I got that one right away. That was a nice ego-booster. :D

    I love prime numbers.

    posted by Allison on 7-8-2009 at 8:58 am

  4. Got it right away. Super easy.

    posted by Laura on 7-8-2009 at 9:35 am

  5. *YES*, my GRE review is paying off, I got that immediately. I’m sure it’s probably the easiest one you guys have ever posted, but thanks so much for the ego boost. It really made my morning:)

    posted by Melissa on 7-8-2009 at 9:41 am

  6. Got it from having used it on my 5th Graders before.

    However, does anyone know the actual formula used in figuring it out, aside from pure guess/test?

    Not trying to stump you, here. It’s a genuine question I have!

    posted by CL Robertson on 7-8-2009 at 10:42 am

  7. Huh. So 101 in a really big font wouldn’t be the largest?

    posted by loomis on 7-8-2009 at 1:39 pm

  8. I got it right, too! No brilliant formula, I just figured that anything ending in 7 is pretty hard to divide by off the top of my head (and yes, I realize there are numbers ending in 7 that aren’t prime), so I went with the largest 3 digit ending in 7!

    posted by C on 7-8-2009 at 6:09 pm

  9. A good formula or method for figuring out smaller prime numbers is to use the Sieve of Eratosthenes. What you do is find the approximate square root of the number in question, then try dividing the original numbers by all the prime numbers up to the square root. For 997, you only need to try primes up to 31. Some, like 2 and 5 can be immediately eliminated. Thus, to show 997 is prime, you only need to divide it by 3, 7, 11, 13, 17, 19, 23, and 29.

    posted by Steve on 7-8-2009 at 6:27 pm

  10. Nice answer Steve. Never knew that.

    posted by SteveO on 7-8-2009 at 6:48 pm

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