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Discrete Maths Rosen Counting

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Total combinations possible are 10.10.10.10 = 10000

If 3 digits are same,

10C1 ways of selecting that number

remaing 1 number = 9C1

and there are 4 ways of placing that single digit in the 4 digit number 

Therefore Totally 10C1 * 9C1 * 4 = 360 Combinations 

But there are chances all 4 digits are same = 10 

If exactly 3 are same, 10000 - 360 = 9640 

 If at least 3 same, 10000 - 360 - 10 = 9630 

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