In all numbers, the middle digit is sum of digits of the product of other two digits
Now, 9 *2 = 18, 1 + 8 = 9 (middle digit in 992)
7 * 3 = 21, 2 + 1 = 3 (middle digit in 733)
8 * 5 = 40, 4 + 0 = 4 (middle digit in 845) and so on.