Let's say you're a parent helping a Grade 1 child with their math, and they're subtracting eight from 17, using small items — counters — like Smarties, multicoloured Rocket candies or Lego ...

For example, 9 = 2 (1 2)+7, and 15 = 2 (2 2)+7. This week's GeekDad Puzzle of the Week is simple: what are the two smallest counter-examples that disprove this conjecture?

The learner might choose a counter different than you. There is no correct answer here — pulling the middle counter down, for example, might make it easier for them to see the remaining nine.