Four Ways to Subtract

Subtracting with nothing decomposed, one ten decomposed, a hundred decomposed across a zero — and the trick that avoids all of it.

Book Add, subtract and round within 1000 3.NBT.A.2 CALF · Subtracting with borrow

▤ Print this All of 3rd grade

For grown-ups

How long: 10 to 15 minutes is plenty. Short and frequent beats long and occasional — the early-numeracy trials with the largest effects ran eight weeks or less.
When they get one wrong: the page shows the working. Read it together rather than moving straight on. Feedback that explains is worth several times more than feedback that only marks right or wrong, and that gap is wider in maths than in any other subject.
Not timed: thinking time is the point of a book. Let silences run.
Print it too: doing the same problems on paper a few days later is worth more than doing twice as many today.

What this practises

Three-digit subtraction across all four cases, and knowing which one you are looking at.

Written subtraction is where the site had the thinnest coverage above grade 2: addition has a game and division has one, and subtraction with regrouping had a single grade-2 holder. The four cases are named rather than mixed silently, because the case a child is looking at determines what they should do first. The compensation case is the one that carries an idea rather than a procedure: adding the same amount to both numbers leaves the difference unchanged, which is why 503 - 298 can be done in your head and 503 - 298 in columns cannot. Interleaving warrant: Rohrer et al. (2020) found interleaved practice beat blocked practice 61% to 38% (d = 0.83) with the same problems and the same total practice, but that trial was grade 7, so applying it here is an inference from mechanism; it is claimed only for the compensation-versus-columns contrast and the word-problem stride, not for the regrouping cases, which differ in difficulty rather than in method.

The underlying idea: A written method is not one skill. The regrouping cases differ enough that a child fluent in one can be stuck in another, and compensation is the case that shows subtraction is about the gap between two numbers rather than about digits.
Where this sits in Illustrative Mathematics: 3rd grade Unit 3: Wrapping Up Addition and Subtraction Within 1,000
Sources: Illustrative Mathematics / Kendall Hunt (2024) · Institute of Education Sciences / What Works Clearinghouse (2021) · Rohrer et al. (2020) all references →

Related measured skill: CALF · Subtracting with borrow. What this means