What number should be in the place of “?”

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

       5
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.

• 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. – Arnaud Mortier Sep 15 '19 at 6:50
• Can you then name one explanation, why the answer would be seven? – No name Sep 15 '19 at 6:59
• What is the source for this puzzle? A textbook, competition, ... ? – Rand al'Thor Sep 15 '19 at 7:03
• Yes, for instance: every number should appear an odd number of times. – Arnaud Mortier Sep 15 '19 at 7:07
• Thanks Arnaud Mortier! Good explanation – No name Sep 15 '19 at 7:10

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 $$15$$, $$?=7$$.