5
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Since my previous cute, easy number puzzle turned out to be a duplicate, I felt I needed to redeem myself.

$2+9 = 6$
$7+8 = 8$
$9+6 = 5$
$0+9 = 6$
$1+4 = 6$
$9+9 = 3$
$2+6 = 6$
$6+4 = 7$
$1+5 = 6$

@TroyAndAbed:
$12+27 = 8$

What does:
$5+5 = ?$
Why?

Hopefully isn't too hard, please ask if you need any test cases.

HINT 1

Use the title of the question

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  • 2
    $\begingroup$ I hope it isn't 10 ;-p $\endgroup$ – Mark N Jul 16 '15 at 18:19
  • 1
    $\begingroup$ If 5+5 isn't 10 I am in trouble for my upcoming 2nd grade math test... $\endgroup$ – Going hamateur Jul 16 '15 at 18:24
  • $\begingroup$ Any chance this uses equivalence classes? $\endgroup$ – Going hamateur Jul 16 '15 at 18:38
  • 1
    $\begingroup$ Is 12 + 27 possible or does it works only with number < 10? $\endgroup$ – TroyAndAbed Jul 16 '15 at 18:43
  • $\begingroup$ @Goinghamateur It doesn't involve equivalence classes. $\endgroup$ – qwertylpc Jul 16 '15 at 19:11
4
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How many total letters are used at least once in the spellings of the words
So 5+5= 4

That is a strong hint

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  • $\begingroup$ Ya, I couldn't think of a more moderate hint $\endgroup$ – qwertylpc Jul 16 '15 at 19:17
  • $\begingroup$ Also I was nervous because of all my down votes :( $\endgroup$ – qwertylpc Jul 16 '15 at 19:17
  • $\begingroup$ Yeah with pure mathematical questions to get upvotes you need to weave it into a story it seems, feels more organic too. $\endgroup$ – Going hamateur Jul 16 '15 at 19:18

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