Can you solve the maths question for Singapore schoolkids that went viral?

http://www.theguardian.com/science/alexs-adventures-in-numberland/2015/apr/13/can-you-solve-the-singapore-primary-maths-question-that-went-viral?CMP=ema_565

I think I got it. It’s keeping the “how does he know that he doesn’t know” question in my head that’s the hard part.

July 16