Subtracting With a Broken Ten

Break the ten first. Then subtract exactly as you did before.

An example

62 − 27. Break a ten: 5 tens and 12 ones. Now 12 take 7 leaves 5, and 5 tens take 2 leaves 3.

The mistake to avoid

Never turn 2 − 7 into 7 − 2. The order matters, and swapping gives the wrong answer.

Remember the tens column has one fewer ten now, because you opened one.

A worked case: 52 - 27

Step 1: not enough ones, so open a ten. 52 becomes 4 tens and 12 ones. Step 2: ones column, 12 - 7 = 5. Step 3: tens column, 4 tens minus 2 tens = 2 tens. Answer: 25.

Remember the tens digit went down

The most common slip is opening the ten to fix the ones and then subtracting the tens as if nothing happened — using 5 tens instead of 4. Cross out the digit and write the new one above it so the change is visible on the page rather than only in your head.

Check before you start

Look at the ones column before doing anything. If the top digit is smaller, you will need to open a ten, and it is easier to do that up front than to discover it halfway through. Deciding first makes the rest mechanical.