If the sequence of the 3 matters, there are (20 x 19 x 18) = 6,840 ways to line up 3 of 20.
If the sequence of the 3 doesn't matter, there are (6,840/6) = 1,140 unique groups of 3.
There are 23C3 = 23!/(20!*3!) = 23*22*21/(3*2*1) = 1,771 combinations.
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.
There are 10 choices for the first number, 9 for the second and 8 for the third. 10*9*8=720 possible combinations.
63
2 x 3 x 3 = 18
There are 6C3 = 20 such combinations.
4
There are 23C3 = 23!/(20!*3!) = 23*22*21/(3*2*1) = 1,771 combinations.
To calculate the number of 4-digit combinations using the digits 1, 3, 5, and 7 exactly once each, we can use the permutation formula. There are 4 choices for the first digit, 3 choices for the second digit, 2 choices for the third digit, and 1 choice for the fourth digit. Therefore, the total number of combinations is 4 x 3 x 2 x 1 = 24. So, there are 24 possible 4-digit combinations using the digits 1, 3, 5, and 7 exactly once each.
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.
The number of combinations is 20C5 = 20!/(15!*5!) = 20*19*18*17*16/(5*4*3*2*1) = 15,504
Combinations of 2: 20*19/(1*2) = 190 Combinations of 3: 20*19*18/(1*2*3) = 1140 Combinations of 4: 20*19*18*17/(1*2*3*4) = 4845 So 6175 in all.
There are 10 choices for the first number, 9 for the second and 8 for the third. 10*9*8=720 possible combinations.
63
2 x 3 x 3 = 18
If their sequence matters . . . 120 If it doesn't . . . 20
Well, honey, there are 20 numbers to choose from for the first digit, 19 for the second, and 18 for the third. So, multiply those together and you get a total of 6,840 possible 3-number combinations. Math can be a real party pooper, but hey, that's the answer for ya!