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For my numbers i chose 4 5 6. I added them together and got 15. I noticed that there is always a factor of three in there, in this one it was 6.

Then for my next set of numbers i chose 8 9 10. I added them together and got 27. And i noticed again that they have a factor of three in there, in this one it was 9.

Yes you can prove that its true because weith any three numbers you just doing the three times table counting three numbers every time. So for examle 11 12 13. The factor of three here is 12 so it will always be a factor of three.