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How many combinations of three digits are there? The answer is 225,
All combinations of 1-9 are 9 * 9 * 9 = 729, and there are 9 * 8 * 7 = 504 combinations without repeated numbers. The number of combinations with repeated numbers can be obtained by subtracting two numbers
(1) How many natural numbers are smaller than 5000 and without repetition? (2) From the nine numbers 1 to 9, five digits are selected each time to form a five digit number without repetition. ① the number on the odd position can only be odd. How many such five digits are there? ② The odd number can only be in the odd position. How many such 5-digit numbers are there?
(1) 4 digits: 4a93 = 2016, 3 digits: 9a92 = 648, 2 digits: 9 × 9 = 81, 1 digit: 10, so there are 10 + 81 + 648 + 2016 = 2755 natural numbers smaller than 5000
15689 five digits make up three digits. How many combinations are there?
There are 60 combinations of 5 × 4 × 3
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