If you include the combination which comprises no numbers (the empty set), then there are 16:
1 with no digits
4 with 1 digit
6 with 2 digits
4 with 3 digits, and
1 with 4 digits.
Since a number can have infinitely many digits, there are infinitely many possible combinations.
Three combinations: 23, 24 and 34
4*3*2*1 = 24 different combinations.
The number of combinations - not to be confused with the number of permutations - is 2*21 = 42.
The number of 4 different book combinations you can choose from 6 books is;6C4 =6!/[4!(6-4)!] =15 combinations of 4 different books.
8C6 = 28
If the numbers can be repeated and the numbers are 0-9 then there are 1000 different combinations.
Number of combinations = 45C6 = 45!/6!(45-6)! = 8,145,060
If every number can be used as many times as you like, there are 104 = 10000 different combinations. If each number can only be used once, there are 9!/(9 - 4)! = 5040 combinations.
221,184 Different Combinations :)
Number of 7 digit combinations out of the 10 one-digit numbers = 120.
all numbers have to be different 720 360 (6*5*4*3*2*1), * * * * * Wrong. Combinations do not have to be different. Those are permutations. There are only 6C4 = 6*5/(2*1) = 15 combinations.