Prime Nine
Probability puzzles require you to weigh all the possibilities and pick the most likely outcome.
There are 9! ways to arrange the digits 1-9 to form a 9-digit number.
E.g. 345, 987, 126
If you know this, then if I wrote all 9! possibilities on cards (1 way per card), and I asked you to select one at random; what is the probability that the number you selected is prime?
HintDivisibility tricks are useful now.
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Answer
Zero.
A well known divisibility rule is that if a number's digits add to 9, the number itself is a multiple of 9.
(E.g. 18, 1+8=9, 18 is a multiple of 9
24786, 2+4+7+8+6=27, 2+7=9, 24786 is a multiple of 9)
Given the fact that 1+2+...+8+9=45, which is a multiple of 9; this means that all 9! numbers are multiples of 9.
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Comments
Scubee   
Jan 11, 2008
| 9 ways to arrange the digits 1-9 to form a 9-digit number?
I'm confused. If my calculations are correct, wouldn't there be 362880 (9*8*7*6*5*4*3*2*1) possible ways to arrange 1-9 into a nine digit number? This doesn't change the answer but confusing still. |
RampaginRobo
Jan 11, 2008
| Scubee, 9! means 9 factorial which is 362, 880.  |
Holo  
Jan 12, 2008
| Actually, there would be 3,486,784,401 ways |
calmsavior
Jan 13, 2008
| Congratulations on having the first approved teaser with a 4 in the ten-thousands position. |
RampaginRobo
Jan 20, 2008
| Firstly, thanks calmsavior! And secondly, where did your idea come from Holo?  |
bookwrmKP
Feb 01, 2008
| Wow. I would have never figured that out.... I guess I'm not as smart as you guys... Very clever! |
jsdodgers
Apr 03, 2008
| One problem. 9! means 9*8*7*6*5*4*3*2*1, not 9+8+7+6+5+4+3+2+1. |
jsdodgers
Apr 03, 2008
| Nevermind my last comment. I understand where you got the 9+8+7+6+5+4+3+2+1 from now. |
scott8anthony
Apr 11, 2008
| I must say this one incorporates a simple rule and disguises it very well; good job! |
stephiesd
May 03, 2008
| Whoo. This teaser was amazing. It actually took me a minute...
But then I got it. (With the hint...  ) |
Lord_Hawk 
Jan 04, 2009
| I like the way this simple problem disguises such a simple division rule. It took me a while to notice it. I must boast that I didn't have to look at the hint though  |
opqpop
Sep 27, 2010
| Too easy.
Which numbers are prime? Oh look, 1+...+9 = 45 which is divisible by 3 and you're done. |
daquity36
Jun 25, 2012
| This problem seems incorrect.. What about #'s such as 127, 137, 149, 157 ... etc?
http://en.wikipedia.org/wiki/List_of_prime_numbers |
daquity36
Jun 25, 2012
| Oops never mind. I thought this problem was for 3 digits. |
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