Talk:2000 AMC 12 Problems/Problem 4

Hello all,

I am reviewing for the AMC 10 and I came across this problem; while the solution says to look at the numbers in mod10, wouldn't it take a really long time to list out all the numbers up until the '6'? Wouldn't you still have to multiply out for every step? Last time I checked, calculators are not allowed on the AMC 10 or 12.

Can someone help?

Thank you!