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

• I hope it isn't 10 ;-p – Mark N Jul 16 '15 at 18:19
• If 5+5 isn't 10 I am in trouble for my upcoming 2nd grade math test... – Going hamateur Jul 16 '15 at 18:24
• Any chance this uses equivalence classes? – Going hamateur Jul 16 '15 at 18:38
• Is 12 + 27 possible or does it works only with number < 10? – TroyAndAbed Jul 16 '15 at 18:43
• @Goinghamateur It doesn't involve equivalence classes. – qwertylpc Jul 16 '15 at 19:11