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?

Like many math students, I had dreams of mathematical greatness. I thought I was close once. A difficult algebra problem in college kept me working late into the night. After hours of struggle, I felt ...