I'm trying to solve this puzzle. Numbers are at triangle, and I have to solve ?-mark:

     6   1
   7       2
  5  ?   7   5

Based on solutions it should be ?=7. But I don't understand the reason for this

I also put pic of it.Triangle quiz

  • $\begingroup$ There could be dozens of answers with a reasonable logic. For instance, it should be 2 as in every little triangle there are two numbers whose sum is the third. $\endgroup$ Sep 15 '19 at 6:50
  • $\begingroup$ Can you then name one explanation, why the answer would be seven? $\endgroup$
    – No name
    Sep 15 '19 at 6:59
  • 1
    $\begingroup$ What is the source for this puzzle? A textbook, competition, ... ? $\endgroup$ Sep 15 '19 at 7:03
  • $\begingroup$ Yes, for instance: every number should appear an odd number of times. $\endgroup$ Sep 15 '19 at 7:07
  • 1
    $\begingroup$ Thanks Arnaud Mortier! Good explanation $\endgroup$
    – No name
    Sep 15 '19 at 7:10

It is:

7. Start with $5,5,5$. The sum is $15$. Move each circle clockwise one step. You now have $1,7,7$, again the sum is $15$. Moving clockwise again you have $2,?,6$. For the sum to be $15$, $?=7$.


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