Well honey, in numbers 1 through 50, each digit from 0 to 9 appears 5 times. It's simple math, darling. Just divide the total number of digits by 10 and you'll get your answer. Now go impress someone with your newfound knowledge, you're welcome!
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It would help to know which digit. 0 appears in 9 numbers and each of the others in 18 numbers.
There are 9 digits that can be the first digit (1-9); for each of these there is 1 digit that can be the second digit (6); for each of these there are 10 digits that can be the third digit (0-9); for each of these there are 10 digits that can be the fourth digit (0-9). → number of numbers is 9 × 1 × 10 × 10 = 900 such numbers.
Each digit can appear in each of the 4 positions. There are 9 digits, therefore there are 9⁴ = 6561 such combinations.
You can select 9 numbers for the first digit, 8 numbers for the second digit, and 7numbers for the third digit; so 504 (e.g. 9*8*7) different three digit numbers can be written using the digits 1 through 9.
10!/3! = 604800 different combinations.