3. (10 points) A project needs 100 days to complete. Initially, 10 people worked for 30 days and completed $\frac{1}{5}$ of the entire project. Then, 10 more people were added to complete the project. How many days earlier can the task be completed? 

Fill in the blank: $\qquad$ days.
【Analysis】First, consider this project as a unit "1". According to the work efficiency $=$ work volume $\div$ work time, use the work volume completed by 10 people in 30 days divided by $10 \times 30$ to find out what fraction of the project each worker completes per day; then find out what fraction of the project 10 more people complete per day, and then according to the work time $=$ work volume $\div$ work efficiency, use the remaining work volume divided by the work volume completed per day by 10 more people to find out how many days the remaining project needs; finally, subtract the actual required time from 100 to find out how many days ahead of schedule the task can be completed.

【Solution】Solution: $100-30-\left(1-\frac{1}{5}\right) \div\left[\frac{1}{5} \div(10 \times 30) \times(10+10)\right]$
$$
\begin{array}{l}
=70-\frac{4}{5} \div \frac{1}{75} \\
=70-60 \\
=10 \text { (days) }
\end{array}
$$

Answer: The task can be completed 10 days ahead of schedule.
Therefore, the answer is: 10.