Two digits+one digit, no carry, that is, one digit plus one digit is less than 10, and one digit must be added with one digit.

Two bits+one bit, carry, that is, one bit plus one bit is exactly 10, one bit is written as 0, and the tenth bit is added with 1. This is the key and difficult point. Use a stick or a counter to make the child understand why the tenth digit of 1 should be entered. The unit is 1 1 ground number, ten is 10 10 ground number,10/yes10.

Two digits+one digit, carry, that is, one digit plus one digit exceeds 10.

I hope it helps you.