5. In the addition problem on the right, the same Chinese characters represent the same digits from 1 to 9, and different Chinese characters represent different digits. Then the two-digit number represented by "数学" in the problem has $\qquad$ possibilities.
$$
\begin{array}{r}
\text { 学习 } \\
\text { 学习 } \\
+ \text { 学习 } \\
\hline \text { 数学 }
\end{array}
$$
Analysis: According to "学+学+学" without carrying, we know that "学" has only 3 possibilities.
"学" =1, "学习" $=17$, "数学" $=51$;
"学" $=2$, "学习" $=24$, "数学" $=72$;
"学" $=3$, "学习" $=31$, "数学" $=93$.
The two-digit number represented by "数学" in the vertical equation has a total of 3 possibilities.