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Cool 6-Digit Numbers

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Let us call a -digit number cool if each of its digits is no less than the preceding digit. How many cool $6-$digit numbers are there?

Details And Assumptions:

  • For example, $,112446  is cool.

  • 233043 isn't cool.

1 Answer

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first we should not that   0 can not be most significant bit

by this we canclude that 0 can not occur any where in cool no

and  this problem can be converted into  combination of 6 object from a set of 9 different object(1,2,3,4,5,6,7,8,9)

9+6-1C 9-1 =14C8=14C6
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