Alex Bellos's Monday puzzle

[citoyen]

I always enjoy these. They are often fiendishly difficult.

Today's is great fun. First few test questions are easy but... then it gets fiendishly difficult:
https://www.theguardian.com/science/2023/jun/26/can-you-solve-it-the-simple-question-almost-everyone-gets-wrong

I've downloaded the Stenciletto app on the back of this. Love it.