Odd Number Out
These brain teasers rely on your ability to recognize groups of common attributes. For each of these puzzles you'll need to figure out why the words or letters are grouped as they are. Sometimes you will be asked to pick the oddoneout or to place a new word into the correct group.
Which is the odd number out?
2460, 3251, 6152, 1836, 2204, 5317
Answer
3251.
The others have the same property: they are composite numbers, and the sum of the first two digits equals the sum of the last two digits.
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Comments
JessicaLynn
Apr 23, 2006
 How can you know if that is a prime number? 
blagh
May 24, 2006
 To work out if a number is prime, work out the square root. Then attempt to divide the number by every prime number below that result. (of course if the square root is a whole number then you know that it's a composite as well.)
for instance, every number from 1  100 can be determined if it's a prime by attempting to divide by 2, 3, 5, 7.
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, and 47 will give you everything from 1  2500.
Of course even numbers, and number that the sum of all digits are divisible by three can automatically be classed as composite, which is 2 out of every 3 numbers. Try dividing by 7 as well and that brings your figure up to roughly 5 out of 7 numbers.
Sorry for the dull explanation, but you did ask. 
blagh
May 24, 2006
 Ooops, left out numbers ending in 5 from that explanation.... Well, I think you get the picture. 
JessicaLynn
Jul 01, 2006
 Yeah! Thanks. 
dshebo
Jun 15, 2007
 Great teaser! really makes you think! (I didn't get it by the way)
Guess I'm not as much of a mathematician as I thought I was. Great explanation though, blagh! 
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