We don't just teach homework answers how to think.

Mr Lee has been correcting homework since before most of his students' parents started school.

He began teaching in MOE schools and has spent the four decades since refining one simple belief: a student who understands why an answer is wrong will never make that mistake again. That's the whole method.

Today he runs small group lessons from home, close to Aljunied MRT, together with Mrs Lee who helps manage scheduling and class materials. Many of his current students are the children of students he taught twenty years ago — which is the only kind of marketing he's ever needed.