  In this paper will be proved the existence of a formula to reduce a tetration
of base $2^{k}$ and $5^{k}$ $\mod 10^{n}$. Indeed, last digits of a tetration
are the same starting from a certain hyper-exponent; In order to compute the
last digits of those expressions we reduce them $\mod 10^{n}$. Lots of
different formulas will be derived, for different cases of $k$ (where $k$ is
the exponent of the base of the tetration). This kind of operation is
fascinating, because the tetration grows very fast. But using these formulas we
can actually have informations about the last digits of those expressions. It's
possible to use these results on a software in order to reduce tetrations $\mod
10^{n}$ faster.
