10. (3 points) Li Gang delivered honeycomb coal to Grandma Wang, a military family member. The first time, he delivered $\frac{3}{8}$ of the total, and the second time, he delivered 50 pieces. At this point, the amount delivered is exactly $\frac{5}{7}$ of the amount not yet delivered. There are still $\qquad$ pieces of honeycomb coal not delivered.
【Solution】Solution: The already transported is exactly $\frac{5}{7}$ of what hasn't been transported, so the already transported is: $\frac{5}{5+7}=\frac{5}{12}$ of the total, and what hasn't been transported is: $\frac{7}{5+7}=\frac{7}{12}$ of the total;
$$
\begin{array}{l}
50 \div\left(\frac{5}{12}-\frac{3}{8}\right) \\
=50 \div \frac{1}{24}, \\
=1200 \text { (pieces); } \\
1200 \times \frac{7}{12}=700 \text { (pieces); }
\end{array}
$$

Answer: There are still 700 pieces not transported.
The answer is: 700.