The question from the 2012 Programme for International Student Assessment (PISA), a test conducted every three years across more than 80 countries in the Organisation for Economic Co-operation and ...

Let's keep things simple – this is basic math. Nothing scary. Just everyday calculations, a bit of geometry, some number patterns, and the kind of stuff you definitely learned in school at some point.

Get your timer out and see whether you can remember the golden rule from grade school math class.