| Please use FileSystem tools to finish the following task: | |
| ### Task Description | |
| Your task is to find all files that contain a substring of 30 or more characters that also appears in `large_file.txt`. **You are not allowed to use python code.** | |
| ### Task Objectives | |
| 1. **Read the reference file** `large_file.txt` to understand its content | |
| 2. **Examine each file** from file_01.txt to file_20.txt | |
| 3. **Find files** that contain a substring of 30 or more characters that matches a substring in `large_file.txt` | |
| 4. **Create a file `answer.txt`** and write the results to it with the following format: | |
| - One line per matching file | |
| - Format: `filename.txt,start_position` | |
| - Where start_position is the character position (1-indexed) of the matching substring in `large_file.txt` | |
| - Do not add any things else other than `filename.txt,start_position`. | |