In Portugal, 11-12 year-olds are in the 6th or 7th grade of compulsory education. Compulsory education starts at 6 years old, but pre-primary education is optional. In 6th and 7th grade, math classes cover first-degree equations, rational numbers, and trigonometry. I think these exercises are interesting, but wouldn't this kind of problem be used to differentiate the more intelligent students in the Singaporean education system? I'm just curious, because it seems a bit advanced for a regular difficulty level.