24 Skidoo

Kevin Chu, of Sun Blog fame, posted a tricky little problem. I'm glad I didn't get this one in an interview!! Don't read the comments attached to Kevin's posting, unless you want to spoil the suspense. If you can't figure it out right away, go do something else. I've often put something down (a computational algorithm, a difficult musical passage on my sax, what appears to be a difficult decision, etc) only to come back to it and the solution or resolution falls right into place. I'm convinced our brains fire off low-priority background threads to ponder unresolved issues. Give it a try. UNIX systems average about 10% utilization. Same with the human brain, according to some researchers. Maybe background processing is consuming some of the spare cycles?

This problem is as easy as it sounds. No tricks at all. You must use all four integers exactly once [1, 3, 4, 6]. You may use each of the four basic arithmetic operators [add, subtract, multiply, divide], and parentheses, as many times as you wish (zero or more times). No other operators are allowed. You should end up with a simple formula that results in exactly 24. Integer math isn't allowed (eg: 6\*4 + 1/3), where 1/3 is truncated to the integer: zero. Good luck!


Comments:

The bit about humans not using their entire brain dates from the 1930's or so, when there were entire sections whose functions were not known. It's not accurate; humans use all of their brain cells on a regular basis. With modern instrumentation, we can accurately measure activity in all parts of the brain, and even watch as a task causes cascades of neuron firings throughout entire regions of the brain.

Posted by David Pipes on March 11, 2005 at 12:00 AM EST #

Fascinating! I'd like to see a documentary on that process. It might also substantiate (or not) the theory that if you are deeply pondering a problem, that your brain continues to churn on it when you are no longer consciously engaged in deep thought (eg: watching a sitcom) :-)

Posted by Dave Brillhart on March 11, 2005 at 01:11 AM EST #

great question. here is another one asked to me years ago, same rules for (1,7,7,7,7) and you will get 100.

Posted by aaa on March 11, 2005 at 06:58 AM EST #

Hmmm. The only solution I can come up with involves one more 7. Using the set [1,7,7,7,7,7], the following results in 100.

(7\*7+1) \* (7/(7+7)) = 50/0.5 = 100 If there is a way with only four sevens, let me know (don't tell me the answer) and I'll keep thinking about it.

Posted by Dave Brillhart on March 11, 2005 at 10:24 PM EST #

yes, there is a solution with 4 sevens, just like the original question; it is logical, no tricks.

Posted by aaa on March 12, 2005 at 01:30 PM EST #

This link is a good starting point: http://faculty.washington.edu/chudler/tenper.html Good links there to follow up on.

Posted by David PIpes on March 14, 2005 at 11:05 AM EST #

the answer is, (7+ (1/7))\*(7+7) = 100

Posted by ahmet on March 21, 2005 at 04:06 AM EST #

Can you please tell me the answer to 24 skidoo. I have a headache from trying to figure it out. I need to know the answer before I die.

Posted by Shannon on September 01, 2005 at 04:12 AM EDT #

The only answer I can come up with is: 1 \*\* 3 \* 4 \* 6 = 24 1 raised to the power of 3 is one, times 4 is 4, times 6 is 24. Or is there another solution I'm not thinking of??

Posted by Brian Graham on September 02, 2005 at 08:16 AM EDT #

The answer is : (6 / ( 1 - 3/4))

Posted by Sree on October 19, 2005 at 11:47 AM EDT #

i got this one

Posted by guest on May 12, 2006 at 12:45 AM EDT #

I suggest another funny solution :-) Let's take numbers are hexadecimal, then, (3-1+4)\*6 = 36 = 24 in hexadecimal

Posted by Yuri Kouxa on February 21, 2007 at 12:26 AM EST #

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