10 More, 10 Less
10 more means tens +1, and the ones digit never changes
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You don't need fingers to add or take away 10: add a ten-rod to the model or remove one, and the tens digit goes up or down by 1 while the ones digit never moves. The number, the tens box and the ones box update together, and little questions like “What is 10 more than 47?” pop up so you can play with the pattern while you calculate.
Tap +10 or −10 to add or remove a ten-rod: only the tens digit changes while the ones stay put, then answer questions like “10 more than 47?”