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?

In many parts of the US, it has long been possible to make local calls with just the seven-digit phone number; the three-digit area code was implicit for calls between numbers using the same area code ...