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March 29, 2015

Posted by **carlton** on Tuesday, April 3, 2012 at 9:06pm.

(a) have exactly three digits which are 9s?

- discrete math -
**MathMate**, Tuesday, April 3, 2012 at 9:17pmThink of 3 9's to which we have to insert a fourth digit x(which is not a nine).

This can be in the four following ways.

X999 or 9X99 or 99X9 or 999X

In each case X can represent 9 possible digits including 0 but excluding 9.

So the number of strings is 4*9=36.

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