This is a story about Newton I read once when I was a child. Now that book is lost and I can only tell you what I remember.
When Newton was young, he had been already famous in curiosity and smart. His family hired a helper. One day, she asked him to go to the market with her because she wasn't good at math. At the market, there was a problem that needed to calculate $3 \times 7$. So the helper asked Newton. After a quick thinking using logarithm, he got the result that $3 \times 7$ must larger than $20$ and smaller than $22$...
So, my question is how did he do that calculate? How to use logarithm to get the result $20<3\times7<22$? Thank you so much.
The story countinues:
... and about to say the result. Before he could finish his math, a near by person had been listened to the conversation and jumped in: "$3$ times $7$ is $21$". "Wow, you are smarter than my Newton", said by the helper. "Indeed, you are smarter than Newton", Newton laughed away.