9+8+2=19 9+7+3=19 9+6+4=19 There are a limited number of possibilities but there are absolutely loads
Number of 7 digit combinations out of the 10 one-digit numbers = 120.
27
There are 15180 combinations.
There are 5,461,512 such combinations.
There are 840 4-digit combinations without repeating any digit in the combinations.
There are 210 4 digit combinations and 5040 different 4 digit codes.
There are 9C3 = 84 combinations.
64
14
There are 5,040 combinations.
There are 1,000,000 possible 6-digit combinations. This is calculated by considering that each digit can range from 0 to 9 (10 possible values), and since there are 6 digits, the total combinations are (10^6 = 1,000,000).
I am assuming you mean 3-number combinations rather than 3 digit combinations. Otherwise you have to treat 21 as a 2-digit number and equate it to 1-and-2. There are 21C3 combinations = 21*20*19/(3*2*1) = 7980 combinations.