(987 + 13)(6 + 4) x 2 x 5 = 1000 x 10 x 10 = 100000
24680
24
4p3*3p1 equal to 72 ways
10 x 9 x 8 = 720.
60
24680
93,876
24
4p3*3p1 equal to 72 ways
18, if leading 0s are disallowed.
0 you can only have 3 layers since you have only 3 numbers so without repeating you would only have 3 layers
10 x 9 x 8 = 720.
There are 840 4-digit combinations without repeating any digit in the combinations.
60
468, 486, 648, 684, 846, 864.
502 205 250 520 4 numbers. If a leading zero is allowed then you can have 052 and 025 for 6 numbers.
There are only six combinations using you rule. They are:- 235, 237, 325, 327, 723 and 725