Answer:
504
Step-by-step explanation:
The left most number must be 1
The second digit can be anything but 1. There are nine numbers to choose from.
The third digit can be anything but 1 and the second digit. There are 8 to choose from
The last digit can by anything but 1, the second digit and the 3rd digit. There are 7 to choose from.
1 * 9 * 8 * 7
504
