6. (10 points) Several computers are used to input a book simultaneously, planned to be completed in several hours. If 3 more computers are added, it will only take $75\%$ of the original time; if 3 fewer computers are used, it will take $\frac{5}{6}$ hours more than the original time. What is the originally planned time to complete the input of this book? ( )
A. $\frac{5}{3}$
B. $\frac{10}{3}$
C. $\frac{5}{6}$
D. $\frac{11}{6}$
【Analysis】Adding 3 more computers, the time is 75% of the original, the efficiency is $1 \div 75\% = \frac{4}{3}$, so originally there were $3 \div \left(\frac{4}{3} - 1\right) = 9$ machines; reducing 3 computers, then 6 computers are used for data entry, the original time is $\frac{5}{6} \times \frac{1}{9} \div \left(\frac{1}{6} - \frac{1}{9}\right)$, solving the problem.
【Solution】Solution: Original number of machines:
$$
\begin{array}{l}
3 \div (1 \div 75\% - 1), \\
= 3 \div \frac{1}{3} \\
= 9 \text{ (units); }
\end{array}
$$

Original time:
$$
\begin{array}{l}
\frac{5}{6} \times \frac{1}{9} \div \left(\frac{1}{9-3} - \frac{1}{9}\right), \\
= \frac{5}{54} \div \frac{1}{18}, \\
= \frac{5}{3} \text{ (hours); }
\end{array}
$$

Answer: The originally scheduled time to complete the data entry of this manuscript is $\frac{5}{3}$ hours.
Therefore, the answer is: $A$.