Thursday, August 20, 2009

Equation using 1, 5, 6, and 7 which equals 21

OK, I finally remember a neat puzzle presented by a college professor while I was still working at Raytheon.

What is the equation which equals 21 and only uses the numbers 1, 5, 6, and 7?

9 comments:

  1. To prevent false solutions the listed numbers can only appear once. 1+1+1..+1 =21 is NOT allowed.

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  2. The answer is.... SPAGHETTI!

    No really, I have no idea. Must your puzzles be math puzzles?

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  3. I was a math major and this one is troubling me. I wonder if Brian could come up with the equation and I am sure it will all be so delightfully simple once it is revealed. Once a magician always a magician.

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  4. It has been a week so here is the answer (there may be additional equations involving square routes).

    6/(1-5/7) = 6/(2/7) = 6*7/2 = 42/2 = 21

    The square route answer is: 6*7/sqrt(5-1).
    This is not considered the correct answer.

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  5. I do not even recognize the symbols used in your equation! How can 1-5 be equal to 2? Is this some kind of witchcraft???? You must have something in error? I think Jennifer's answer spaghetti works well.

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  6. The equation is in standard evaluation form.
    The numerator is 6
    The denominator is 1 - (5/7) which is 2/7
    (The above line is also (7-5)/7)
    Evaluated as 6 / (2/7) = 6*7/2 = 21

    Remember, multiply and divide left to right before addition and subtraction.

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  7. Thanks. I still like spaghetti for the answer....

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  8. Well now that I see the answer, it all seems so simple. Post a new puzzle!!!

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  9. I think you gave me this problem about a year ago. Another puzzle!!11!!1

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