2. There are 1993 boxes arranged in a row from left to right, each containing some small balls. If the leftmost box contains 7 balls, and every four adjacent boxes contain a total of 30 balls, then the rightmost box contains $\qquad$ balls.
2. 7

Let the number of balls in the small boxes from left to right be
$$
\begin{array}{c}
7, a_{2}, a_{3}, \cdots, a_{1993}, \\
\because \quad 7+a_{2}+a_{3}+a_{4}=30, a_{2}+a_{3}+a_{4}+a_{5}=30, \\
\therefore \quad a_{5}=7 .
\end{array}
$$

Similarly, $a_{9}=a_{13}=a_{17}=\cdots=a_{4 k+1}=a_{1993}=7$.