Answer:
Well, a flashback is a memory. Mostly used in stories. A flash back from 20 years is probably someone thinking about something they did 20 years ago. I'll put an example in the explanation.
Step-by-step explanation:
50 year old Harriet began to walk down Country Lane. Suddenly she remembered what had happened 20 years ago on that same lane. She was 30 years old and had just finished eating dinner with her boyfriend Frank. They had known each other for 10 years. That was when Frank had proposed. (here you put every detail by the way) Harriet closed her eyes and felt herself go back to reality. Frank was gone. And so would she be in a few years...