1968 AHSME Problems/Problem 21
Problem
If 
, then the units' digit in the value of S is:
Solution
Note that every factorial after 
has a unit digit of 
, meaning that we can disregard them. Thus, we only need to find the units digit of 
, and as 
mod 
, which means that the unit digit is 
, we have our answer of 
as desired.
See also
| 1968 AHSC (Problems • Answer Key • Resources) | ||
| Preceded by Problem 20 |
Followed by Problem 22 | |
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| All AHSME Problems and Solutions | ||
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions. 
