For x positive integers, function f(x) which remains a mystery and nobody knows can be divided by all of the following five EXCEPT one number. 1, 2, 3, 4 and 6. Can you figure out which number cannot divide f(x)?


closed as off-topic by Glorfindel, JonMark Perry, boboquack, Rubio Apr 15 '18 at 9:52

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  • $\begingroup$ This appears to be a basic mathematical problem, not a puzzle. Is this homework? $\endgroup$ – xhienne Apr 15 '18 at 8:48
  • $\begingroup$ What math subject gives this as a homework? Do enlight me. $\endgroup$ – Kenneth Kho Apr 15 '18 at 8:55
  • $\begingroup$ Prime factorization? I don't know. I didn't mean to hurt you. What is a homework in one country may not be one in yours. Look for math puzzles on YouTube, you'll discover math problems that have been given to pupils that are really hard, even for adults, and you'll wonder how maths are being taught in those countries. $\endgroup$ – xhienne Apr 15 '18 at 9:13

The answer is:



Any function can be divided by 1. So, we eliminate 1. Next, if the function couldn't be divided by 2, then, it also would not be divisible by 4 or 6 which violates our condition. This, 2 is eliminated. Next, by similar logic, 3 is also eliminated. This, we know for sure that f (x) is divisible by 1,2 and 3. As, it is divisible by both 2 and 3, it would be divisible by 6 too. Thus, the only remaining number by which it is NOT divisible is 4.

  • 1
    $\begingroup$ Yep, you got it. "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."-Sherlock Holmes $\endgroup$ – Kenneth Kho Apr 15 '18 at 8:55
  • $\begingroup$ @KennethKho If this is correct, please mark it as correct by clicking the green tick mark next to the answer. $\endgroup$ – F1Krazy Apr 15 '18 at 20:25

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