Your first guess is likely wrong. You’ve been warned.

The numbers in the following graph are arranged logically, according to a simple rule. Your job is deduce the rule and fill in the missing number. No advanced math is required.

If you get the answer quickly, it’s probably wrong - check your work on the whole graph. If you don’t get it right away, let it roll around your mind for a while and come back to it every so often. When you least expect it, the answer will come to you, bringing with it an almost mystical joy.

Today’s puzzle was created by Japanese puzzle wizard Nob Yoshigahara and I learned about it in this wonderful new book by Alex Bellos:

Can You Solve My Problems?: Ingenious, Perplexing, and Totally Satisfying Math and Logic Puzzles