The first number is laid out on a place-value mat as hundred flats, ten rods and one cubes. The student taps a rod to break it into ten cubes (or a flat into ten rods) until every column has enough to take away, then types the answer.
The ones always need regrouping, the tens about half the time, and sometimes there are no tens at all, so a hundred has to be broken first. The answer is only marked right once the mat could really take the subtraction away.
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