In decimal, 4 digits ( 0 - 9) you can represent any number from 0000 to 9999. That is 10,000 unique number combinations. Incidentally, the number of unique cominations is 10 to the 4th power. In binary, 4 digits (0 - 1) can represent 16 different cominations from 0000 - 1111. This is given by 2 to the 4th power
27 three digit numbers from the digits 3, 5, 7 including repetitions.
24
There are 5*4*3 = 60 such numbers.
A lot. And there is akways the code. 1234
Five numbers (5, 6, 7, 8, 9) can be selected for each of the 5 digits. So, there are 5*5*5*5*5 or 55 or 3125 five digit numbers.
5040 different 4 digit numbers can be formed with the digits 123456789. This is assuming that no digits are repeated with each combination.
757
720 (10*9*8)
823543
625 if numbers can have leading 0s, 500 otherwise.
9x9x8x7x6x5x4x3 =1632960
There are 5! = 120 such numbers.
81
9x9x8x7x6x5 = 81x56x30 = 136080
There are 4 possible numbers if the digits are not repeated; 18 if they are. Those are 3-digit numbers, assuming that zero would not be a leading digit. If zero is allowed for a leading digit, then you can have 6 for the non repeated, and 27 if repetition.
162
27 three digit numbers from the digits 3, 5, 7 including repetitions.