2010 AMC 10A Problems/Problem 24
Problem
The number obtained from the last two nonzero digits of is equal to . What is ?
Solution 1(Bigbrain)
We will use the fact that for any integer ,
First, we find that the number of factors of in is equal to . Let . The we want is therefore the last two digits of , or . Since there is clearly an excess of factors of 2, we know that , so it remains to find .
We can write as where where every number in the form is replaced by .
The number can be grouped as follows:
Hence, we can reduce to
Using the fact that ,we can deduce that . Therefore .
Finally, combining with the fact that yields .
Solution 2(bash)
First, we list out all the numbers
Since we must get rid of ending s, we get rid of and the corresponding
Next, we note that ,, and , so it can be simplified to
See also
2010 AMC 10A (Problems • Answer Key • Resources) | ||
Preceded by Problem 23 |
Followed by Problem 25 | |
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