SOLUTION: how many number can be formed from the digit 1,2,3,9, if repetition of digit is not allowed

I can choose the first digit any of 4 ways, a 1,2,3, or 9

For each of the 4 ways I chose the 1st digit, I can choose the 2nd
digit any of 3 ways, any of the 3 digits that I didn't chose for the 
1st digit.  That's 4×3 or 12 digits

For each of the 4×3 or 12 ways I chose the 1st 2 digit, I can choose the 3rd
digit any of 2 ways, any of the 2 digits that I didn't chose for the 
1st 2 digits.  That 4×3×2 or 12×2 or 24 ways to chose the first 3 digits.

That only leaves one digit to choose for the 4th digit, so we just put it on
each of those 24 and that means that there are 4×3×2×1 or 24 ways to choose
the 4-digit numbers.  Here are all 24 numbers that can be made with digits
1,2,3 and 9 with no repetitions of digits:

 1.  1239
 2.  1293
 3.  1329
 4.  1392
 5.  1923
 6.  1932
 7.  2139
 8.  2193
 9.  2319
10.  2391
11.  2913
12.  2931
13.  3129
14.  3192
15.  3219
16.  3291
17.  3912
18.  3921
19.  9123
20.  9132
21.  9213
22.  9231
23.  9312
24.  9321

Edwin