Question 891827: How many possible 4-digit combinations can be obtained using only the digits 1,3, and 9? List the combinations.
Answer by Edwin McCravy(20056)   (Show Source):
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Choose the 1st digit any of these 3 ways 1,3, or 9
That's 3 ways to choose the 1st digit.
For each of those 3 ways to choose the 1st digit, we
may also choose the second digit any of these 3 ways
1,3, or 9 as well.
That's 3*3 or 9 ways to choose the 1st 2 digits.
For each of those 3*3 or 9 ways to choose the first 2
digits, we may also choose the 3rd digit any of these
3 ways 1,3, or 9 as well.
Thats 3*3*3 or 9*3 or 27 ways to choose the first three
digits.
For each of those 3*3*3 or 27 ways to choose the first 3
digits, we may also choose the 4th digit any of these
3 ways 1,3, or 9 as well.
Thats 3*3*3*3 or 27*3 or 81 ways to choose the 4 digits.
digits.
1. 1111
2. 1113
3. 1119
4. 1131
5. 1133
6. 1139
7. 1191
8. 1193
9. 1199
10. 1311
11. 1313
12. 1319
13. 1331
14. 1333
15. 1339
16. 1391
17. 1393
18. 1399
19. 1911
20. 1913
21. 1919
22. 1931
23. 1933
24. 1939
25. 1991
26. 1993
27. 1999
28. 3111
29. 3113
30. 3119
31. 3131
32. 3133
33. 3139
34. 3191
35. 3193
36. 3199
37. 3311
38. 3313
39. 3319
40. 3331
41. 3333
42. 3339
43. 3391
44. 3393
45. 3399
46. 3911
47. 3913
48. 3919
49. 3931
50. 3933
51. 3939
52. 3991
53. 3993
54. 3999
55. 9111
56. 9113
57. 9119
58. 9131
59. 9133
60. 9139
61. 9191
62. 9193
63. 9199
64. 9311
65. 9313
66. 9319
67. 9331
68. 9333
69. 9339
70. 9391
71. 9393
72. 9399
73. 9911
74. 9913
75. 9919
76. 9931
77. 9933
78. 9939
79. 9991
80. 9993
81. 9999
Edwin
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