A Math Problem Illustrates the Difference in Educational Quality

A high school math problem from Singapore stymies adults:

http://www.theatlantic.com/education/archive/2015/04/the-math-question-that-went-viral/390411/

Solution: http://www.nytimes.com/2015/04/15/science/answer-to-the-singapore-math-problem-cheryl-birthday.html 

It also explains why students from Singapore typically finish at the top of the international PISA tests.