MathNet 2010 — Part 07 Question 114

How many nondecreasing sequences $a_{1}, a_{2}, \ldots, a_{10}$ are composed entirely of at most three distinct numbers from the set $\{1,2, \ldots, 9\}$ (so $1,1,1,2,2,2,3,3,3,3$ and $2,2,2,2,5,5,5,5,5,5$ are both allowed)?