This Clever Math Trick Makes It Easy to Check Your Work

Math can be tricky sometimes. Let's say you're adding several large numbers

a

together, and you get an answer that you think is correct. How do you make sure? The simplest way to check your answer is to do the calculation again, but this is tedious and time-consuming. Fortunately, there's a better way, called Casting Out Nines.

Youtuber Numberphile explains how the method works, with the help of mathematician James Grimes: