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Possible Duplicate:
The last two digits of $9^{9^9}$

How to find the last two digits of $9^{9^{9^9}}$ (a power tower of 4 $9$'s) ?

Is there any special approach to these kind of problems?

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    Related: [How do I compute $a^b\,\bmod c$ by hand?](http://math.stackexchange.com/questions/81228)2015-08-20

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