Hit Points Calculation Explanation

I was so confused today. I was looking at a Black Dragon, which is not all that unique with what I was confused about. So the base black dragons basic stats are the following:

CR 3; Size Tiny; Hit Dice 4d12

Speed 60 ft., swim 60 ft.

Natural Armor +3; Breath Weapon line, 2d6 acid

Str 11, Dex 16, Con 13, Int 8, Wis 11, Cha 8

So base Hit dice are 4 d12, great. I'm all good. So I want to move up to a Young Black dragon, and I refer to the Dragon Age Categories table. When you move up to the Young Black Dragon the hit dice column is Base + 4, Alright, so that is 4d12 + 4d12 and that makes it 8d12, wait a second, what is that +24 after the 8d12? This is where I was confused. I scoured the Bestiary, the Core Rule Book, The Beginner Books, and nada. I finally turned to the forums. What do you know, quick answer on the forums.

AC 21, touch 12, flat-footed 19 (+2 Dex, +9 natural)

hp 76 (8d12+24)
Str 19, Dex 14, Con 17, Int 10, Wis 13, Cha 10

So it turns out if you take your Con of 17, and use the modifier: +3 and multiply that by the number of hit dice (8) you get 24 which is the extra hit points I was missing. So you use the average for the hit dice which is 6.5 and multiply that by the number of hit dice, 8, and then add the Con bonus times the number of hit dice and total that.

The equation is as follows

(Number of hit dice x Average of the die) + (Number of hit dice * Con bonus)

So:

(8 x 6.5) + (8 x 3) = 76

52 + 24 = 76