A stranger arrived at a small town of 50,000 people at 8 o'clock in the morning. He had some important news that everyone wanted to hear.
.In the first house he stopped at he told 3 people the news then continued his journey. So at 8:15 a.m. only 3 people in the town had heard the news.
The 3 people all hurried off to each tell three new people the news. This takes them a quarter of an hour. As each new person hears the news, they pass it on to 3 more new people, which takes 15 minutes, and so on.