When I was in middle school, I had a book (in Chinese), teaching children how to carry out some simple calculations fast. It had a back story, with village boys

When I was in middle school, I had a book (in Chinese), teaching children how to carry out some simple calculations fast. It had a back story, with village boys trying to help out the adults when they're stuck with such simple calculations. It mentioned some interesting facts and I can still remember them today. Like when you divide anything by 7, you get a permutation of the string 142857 as the fractional part. Also notice how 14, 28 and 57 (yes, I mean it) are multiples of 7.