They're teaching you wrong

Overheard at a restaurant tonight:
Mother: So, what did you learn in school today?
Son: (Excited) We learned to do one takeaway one!
Mother: Really? What is one takeaway one?
Son: (with obvious pride) Zero!
Mother: Are you sure?
Son: (slightly flustered) Uh, yeah ... Zero ...?
Mother: One takeaway one is ten. They're teaching you wrong, aren't they?