18.
(1)
The probability distribution of $X$ is:
$$\begin{gathered}
P ( X = 0 ) = 0.2 \\
P ( X = 20 ) = 0.8 \times ( 1 - 0.6 ) = 0.32 \\
P ( X = 100 ) = 0.8 \times 0.6 = 0.48
\end{gathered}$$
(2)
Given that type A questions are answered first, the mathematical expectation is...