10. Define $a * b=3a-b$. The solution to the equation $2 *(5 * x)=1$ is ( ).
(A) $\frac{1}{10}$
(B) 2
(C) $\frac{10}{3}$
(D) 10
(E) 14
10. D.
$$
\begin{array}{l}
\text { Given } 2 *(5 * x)=1 \Rightarrow 2 *(3 \times 5-x)=1 \\
\Rightarrow 3 \times 2-(15-x)=1 \Rightarrow x=10 .
\end{array}
$$