There is a game (sort of) called Challenge 24. Basically, you get 4 numbers that you must make equal 24, using each one only once, and only using +, -, /, and *, the four basic operations:
Poster 1: 3, 6, 1/4, 3.
Poster 2: (6/3)*(3/(1/4))=24
5, 7, 1/8, 2/3 etc.
First four:10, 7, 6, 2.
Wonderful game. Brings back good memories. A little to easy if you aren’t in direct timed competition though.
Anyways…
(10 - 7) * (6 + 2) = 24
And I’ll contribute:
5, 23, 7, 6
7 * 6 + 5 - 23 = 24
Next up:
13, 3, 1, 9
13+3+9-1=24
27, 2, 9, 16
13 + 3 + 9 - 1 = 24
Interesting…
Here ya go:
2 3/4, 2, 9, 6
Er, two?
Odinoneeye: (27 / 9) * (16 / 2) = 24
Hal Briston: 2.75 * 2 * 6 - 9
And the new on is: 12, 9, 7, 5
And you thought this would be too easy, Kluge. If it proves to be too easy, we can always change the rules slightly. I’m surprised the math guys aren’t in here, though… This sort of thing should be right up their alley.
Anyway, 7*5-12-9=24. That took me so long to figure out. I spent about half an hour on it and gave up, and the answer came to me while this reply page was loading. (Who else misses the old format with the reply box at the bottom of the thread?)
3, 5, 2, 6
I keep looking at 7*5-12-9 and coming up with 14
352-6=24
How about 21,6,8,2
21 + 6 / (sqrt(8 / 2))
7, 9, 11, 4.5
Whoops, I didn’t see that the rules specified only ±*/. That makes it a lot harder, and I can’t find a solution for 21, 6, 8, 2 at all.
(Although if we’re allowed to use “ln” to mean log-base-2, there’s 21 * 2 - 6 * ln(8) )
Didn’t mean to come up with a show stopper. My Bad. I just chose some random numbers.
Do I win something? 
In order to get thinkgs back on track, how about:
2 10 13 4
(13-10)24 =24
Now try 7, 7, 3, 3
This one is possible, I guarantee it!
Took me a while, but (3+3/7)*7=24.
Probably not too hard: 3, 7, 5, 2
OK, I decided to ruin everything by writing a computer program to solve problems of this sort…
First of all, 21, 6, 8, 2, is in fact unsolvable. The closest one can come is:
((21)-(6))+((8)+(2)) = 25
((21)/(2))+((6)+(8)) = 49/2
((21)+(2))+((8)/(6)) = 73/3
((21)+(2))+((6)/(8)) = 95/4
(21)+((2)/((6)/(8))) = 71/3
((21)+(6))-((8)/(2)) = 23
As for 3, 3, 7, 7:
(7)*((3)+((3)/(7))) = 24
Oh, and 7,9,11,4.5 has no solution either
Anyhow, with the massive power of this computer program, I have come up with the following question: using 1, 2, 3, and 4 as your 4 numbers, what is the smallest integer which can NOT be generated in this method?
(3 * 5) + 7 + 2 = 24
How about 3, 3, 7, 12 ?
((3*3)-7)*12=24
2,14.4,3,10
(14.410)/(23) = 24
This next one is designed for MaxTheVool to specifically
test his massively-powerful computer program (sic):
1002 1528 12345 995141083160
---- , ---- , ----- , ------------
3001 6779 67899 76867982607
(Thanks, Mathematica 5 )
[14.4/(2*3)] * 10 = 24
4.8, 2, 8, 1