12, 13, 14, 21, 31, 41: 6 if repetition is not allowed.
Sure. Any 1-digit number contains only one-digit numbers as factors. Any larger number, on the other hand, contains itself as a factor, so these can be excluded.
16000
5040 numbers can be made.
There are 9 x 10 x 10 x 10, or 9000 four-digit number that can be formed if the leading digit cannot be zero.
1339
Sure. Any 1-digit number contains only one-digit numbers as factors. Any larger number, on the other hand, contains itself as a factor, so these can be excluded.
16000
The number 1234567890 can create about 39 of 40 4 digit numbers.
Choose from 4 for the first number, 3 for the second number, and 2 for the third number. Therefore there are 4*3*2 = 24 three digit numbers can be formed from 3769 if each digit is used only once.
5040 numbers can be made.
Since both of those numbers contains four digits, there are no three-digit numbers between them.
There are 9 x 10 x 10 x 10, or 9000 four-digit number that can be formed if the leading digit cannot be zero.
3
There are 2000 such numbers.
Only four numbers:20233032
It is 120 if the digits cannot be repeated.
1339