Multiply the number of possible starting numbers by the number of possible middle numbers by the number of possible end numbers to get your result.
In example, The possible starting numbers are 1, 2, 3, 4, 5, 6, 7, 8, and 9. Let's say I picked nine as the starting number. Since your question states that each number can only be used once, we eliminate nine from the selection of middle and end numbers. Now, the choices for the possible middle and end numbers are 1, 2, 3, 4, 5, 6, 7, and 8.
Possible starting numbers= 9
Possible middle numbers= 8
Multiply 9 by 8. You get 72 different choices for a two digit number.
Let's say that the middle number I picked was two. We then remove the number two from the possible choices for the final numbeselections: 1, 3, 4, 5, 6, 7, and 8.
Possible outcomes for two digit number= 72 (which is 9 times 8)
Possible end numbers = 7
Multiply 72 by 7 to get the possible outcomes for a three digit number with each digit used only once.
72 times 7 = 504. You have 504 possible outcomes.
A33 =3x2x1=6 234 243 324 342 432 423
5040 different 4 digit numbers can be formed with the digits 123456789. This is assuming that no digits are repeated with each combination.
24 different numbers.
There are 5 numbers of 1 digit, 25 numbers of 2 digits, and 75 numbers of 3 digits. This makes 105 numbers in all.
There are 3 values (1, 2, 3) for each of the 4 digits. Therefore, there are 3*3*3*3 or 81 four digit numbers that can be formed.
18
There are 9 digits that can be the first digit (1-9); for each of these there is 1 digit that can be the second digit (6); for each of these there are 10 digits that can be the third digit (0-9); for each of these there are 10 digits that can be the fourth digit (0-9). → number of numbers is 9 × 1 × 10 × 10 = 900 such numbers.
20
If the digits may not be repeated, there are 6x5x4x3x2x1 = 720 numbers, If digits can be repeated, the answer is 6x6x6x6x6x6 = 46,656 numbers If zero is ong the available digits but is excluded from the first place. 5x6x6x6x6x6=38,880 numbers
5 x 10 x 5 = 250 different numbers, assuming there is no limit to each digits' use.
9630
If they can be repeated, 343 different numbers. If they can be used once each, there are only 210.
Four digit numbers span from 1000 to 9999. This totals (9999-1000) +1 = 9000 different four digit numbers