-59
Without duplications, the answer is
60C6 = 60*59*58*57*56*55/(6*5*4*3*2*1) = 50,063,860
6
10
Six combinations: 123, 132, 213, 231, 312, 321
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.
Only one.
45
6
10
32C3 = 4960
Six combinations: 123, 132, 213, 231, 312, 321
Assuming you are treating each number as a number and not as an individual unit, the numbers you can make from these digits are 899, 989 and 998.
There are 167960 9 digits combinations between numbers 1 and 20.
Assuming the digits cannot be repeated, there are 7 combinations with 1 digit, 21 combinations with 2 digits, 35 combinations with 3 digits, 35 combinations with 4 digits, 21 combinations with 5 digits, 7 combinations with 6 digits and 1 combinations with 7 digits. That makes a total of 2^7 - 1 = 127: too many for me to list. If digits can be repeated, there are infinitely many combinations.
10,000.
9,000 - all the numbers between 1,000 and 9,999 inclusive. * * * * * NO. Those are PERMUTATIONS, not COMBINATIONS. Also, the question specified 4 digit combinations using 4 digits. The above answer uses 10 digits. If you start with 4 digits, you can make only 1 combination.
∞ \ Infinite
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.