The reasons are as follows: this is a ranking problem, which can be calculated step by step. There are eight possibilities for thousands: 2-9 * * *; There are seven possibilities for the number in hundreds (the number in thousands just used is no longer available). Similarly, there are six possibilities and five possibilities for numbers in decimal system, so all possibilities * * * have 8*7*6*5= 1680 different possibilities.
Besides, I sympathize with you! But there is no way!