factorial problem

@Ketan: I also know that method which u r talking about . find the prime factors divide by10^19. . then taking the last digit of the remaining factors, multiplying them and getting the last digit of the product as the answer to the q.

hmmm.
take modulo 10 after removing the powers of 10.

visit this link.

http://www.mathpages.com/home/kmath489.htm

here is a past year problem in AMC and its solution.
http://www.artofproblemsolving.com/Wiki/index.php/2010_AMC_10A_Problems/Problem_24