Which number should go in the empty triangle?

From one of my favourite maths teaching web sites, subtangent.com/mathsAuthor - Duncan Keith 2004

Can you answer the question that has gone viral from the admissions test at Hong Kong Elementary School?  At age 6 they are expected to do this in 20 seconds.  I must admit - it took me a little more than 20 seconds!

I still haven't found anyone (including teachers) who can do this without a hint- can you?