Twin socks
You know 6 + 6 = 12 by heart, so 6 + 7 is just 'one more' — 13. Doubles are facts your brain grabs instantly, and near-doubles ride along for free.
Doubles as anchors
Memorize the doubles (7+7=14, 8+8=16). Then a near-double like 7+8 is the double plus one: 15.