This mathematical trick lets you work out the weekday of any date

Time comes in cycles, and we deal with this by using a cyclic number system. What time is 5 hours from 10pm? Hopefully you’d say 3:00, not 3:00. You know that 10 + 5 is not 3, but you know that it resets to zero when you press 12. mathematical termsthis is called modular arithmetic: we divide 15 by 12 and keep the remainder. We write this as 10 + 5 = 3 modulo 12.

Modular arithmetic has many applications, from cryptography to magic tricks. There is…