3. 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?
【Answer】 10
【Analysis】Assuming the work efficiency of each person per day is $a$ units, the total amount of work is $10 a \times 30 \div \frac{1}{5}=1500 a$ (units); after adding 10 more people, the number of days to complete the work is: $(1500 a-30 \times 10 a) \div(10 a+10 a)=60$ (days) ahead of schedule by $100-30-60=10$ (days) to complete.