Question 24
Let $a_1,a_2,a_3,a_4,a_5,a_6,a_7,a_8,a_9$ be distinct non-zero digits so that $a_1\lt a_2\lt a_3\lt a_4\lt a_5$, $a_1\lt a_6$, $a_2\lt a_7$, $a_3\lt a_8$ and $a_4\lt a_9$. How many different ways are there to assign the values to $a_1,a_2,a_3,a_4,a_5,a_6,a_7,a_8,a_9$?