The crisp new dollar bill that I have just taken from my wallet bears the serial number 24598176. It’s easy to tell that the number is exactly divisible by 2 but not by 5. Is it divisible by 3? by 4?

When I was a little kid I had a friend who was born on February 29, the “leap day” we add to that month every four years. I remember we used to tease him by saying that he was only three years old. I ...

This article is a modified and updated version of one I wrote in—oddly enough—2008 and then updated for 2012. Barring a colossal asteroid impact or a Trump presidency, I’ll probably be around to do it ...

A: A tropical year is the time (365.242199 days) the earth takes to revolve around the sun once. By having just 365 days in a calendar year, 0.242199 days are lost each year. To correct this error, ...