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# Counting all 2-distinct-digit occurrences from 4-digits combinations

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Joined: 02 Nov 2017
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Counting all 2-distinct-digit occurrences from 4-digits combinations [#permalink]

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02 Nov 2017, 11:13
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Currently I'm facing a problem to find out the formula to count 2 distinct digits say X and Y from a 4-digit combinations, where each digit can be range from 0 to 9, programming solution give me an answer of 974

I'm able to find out if to count 1 digit say X from a 4-digit combinations: $$10^4 - 9^4 = 1000 - 6561 = 3439$$

I've tried to read some articles, but a bit too dumb to understand how it works. I'm trying to get my colleague help too, it seem not so easy to get the formula, also not sure the programming answer is correct.

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Math Expert
Joined: 02 Sep 2009
Posts: 42258

Kudos [?]: 132703 [0], given: 12335

Re: Counting all 2-distinct-digit occurrences from 4-digits combinations [#permalink]

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02 Nov 2017, 23:35
whislai wrote:
Currently I'm facing a problem to find out the formula to count 2 distinct digits say X and Y from a 4-digit combinations, where each digit can be range from 0 to 9, programming solution give me an answer of 974

I'm able to find out if to count 1 digit say X from a 4-digit combinations: $$10^4 - 9^4 = 1000 - 6561 = 3439$$

I've tried to read some articles, but a bit too dumb to understand how it works. I'm trying to get my colleague help too, it seem not so easy to get the formula, also not sure the programming answer is correct.

_________________

Kudos [?]: 132703 [0], given: 12335

Re: Counting all 2-distinct-digit occurrences from 4-digits combinations   [#permalink] 02 Nov 2017, 23:35
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