A positive integer k is called a "lucky" number if the units digit of k^6 is 4. How many lucky numbers smaller than 1000 are there?

Guest Apr 6, 2021

#2**+1 **

$k^6\equiv 4\pmod{10}$ has solutions $k\equiv \pm 2\pmod{10}$.

So $2,8,12,18,22,28, ...$ all work.

thedudemanguyperson Apr 6, 2021