The number of natural numbers, between 212 and 999, such that the sum of their digits is 15, is _____.

Let x = 2 $\Rightarrow$ y + z = 13
(4,9), (5,8), (6,7), (7,6), (8,5), (9,4),  6
Let x = 3$\to$y + z = 12
(3,9), (4,8), ........, (9,3) $\to$7
Let x = 4 $\to$ y + z = 11
(2,9), (3,8), ........, (9,1) $\to$ 9
Let x = 5 $\to$ y + z = 10
(1,9), (2,8), ........, (9,1) $\to$10
Let x = 6 $\to$ y + z = 9
(0,9), (1,8), ........, (9,0) 9
Let x = 7 $\to$ y + z = 8
(0,9), (1,7), ........, (8,0) $\to$9
Let x = 8 $\to$ y + z = 7
(0,7), (1,6), ........, (7,0) $\to$ 8
Let x = 9 $\to$ y + z = 6
(0,6), (1,5), ........, (6,0) $\to$ 7
Total = 6 = 7 + 8 + 9 + 10 + 9 + 8 + 7 = 64