19.2. What is the last digit of the sum

$$
3^{13}+3^{15}+3^{17}+\ldots+3^{43} ?
$$
19.2. There are 16 addends in total in this sum. By sequentially dividing them into 8 pairs, we get:

$$
\begin{gathered}
3^{13}(1+9)+3^{17}(1+9)+\ldots+3^{41}(1+9)= \\
=\left(3^{13}+3^{17}+\ldots+3^{41}\right) \cdot 10
\end{gathered}
$$

Thus, the considered sum ends in 0.