Teens Are Ten and Some
A teen number = 1 ten and some ones
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Teen numbers look tricky, but every one of them is just “one ten and some ones”. A bundle of exactly 10 sits on the left and loose ones on the right: tap +1 / −1 to change the loose ones and watch the teen number, its tens digit and ones digit move together — the foundation for place value and column arithmetic.
The left bundle holds exactly 10, the right side has loose ones. Tap +1 / −1 to change the loose ones and watch how every teen number is one ten plus some ones — the tens and ones digits update together.