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 the joy of discovery.
Today’s puzzle was created by Japanese puzzle wizard Nob Yoshigahara and I learned about it in this wonderful book by Alex Bellos: