Subtracting to 100

Take away in parts

57 − 23. It looks scary. It counts beautifully.

57 is 5 tens and 7 ones. I take away 23, so 2 tens and 3 ones.

Tens: 50 − 20 = 30. Ones: 7 − 3 = 4. Left is 30 + 4 = 34.

The same song as with adding: crates to crates, bottles to bottles.

What is it for? Pocket money. You have sixty Kč, you spend twenty-three — how much is left? That is exactly this counting. Anyone who can subtract to a hundred always knows how much is left in their purse. And nobody in a shop can cheat them. The method is the same as with adding, you just take away.

Just one thing is tricky: what if you do not have enough ones? 42 − 5, but two is smaller than five. Do not worry. We have the old stop at ten for that. We will show both cases nicely in order.

Jonáš is saving for a building set. He has 65 Kč. He buys his brother a present for 24 Kč. “How much do I have left?” Sofie advises him in parts: “First take away two 10 Kč coins.” Jonáš puts two 10 Kč coins aside: 45 is left. “And now four Kč.” Jonáš counts four Kč: 41. “Sixty-five minus twenty-four is forty-one,” he writes. Then he counts the coins that are left. Exactly 41 Kč. “In parts it is like two small jobs instead of one big one,” he says happily. He still has a bit to save for the building set. He writes the leftover in a notebook so tomorrow he knows where he stopped.

Subtract in parts, watch the ones

Easy case: there are enough ones. 57 − 23:

1. Take away the tens: 57 − 20 = 37.
2. Take away the ones: 37 − 3 = 34.

Done. 57 − 23 = 34.

Tricky case: there are not enough ones. 42 − 5. Two is not enough for five. Use a stop at ten, like with subtracting to 20:

1. Take away ones to a full ten: 42 − 2 = 40.
2. Split the five: 5 = 2 + 3. 3 are left to take away.
3. 40 − 3 = 37.

So 42 − 5 = 37.

And together? 63 − 28: take away the tens — 63 − 20 = 43. Now take 8 from 43: three is not enough, so through ten — 43 − 3 = 40, then 40 − 5 = 35. In all 63 − 28 = 35.

The check is still the same and still just as strong: difference + taken away = original. 35 + 28 = 63? Tens 30 + 20 = 50, ones 5 + 8 = 13, in all 63. It matches.

78 − 34 = ?

There are enough ones (8 against 4). Easy case.

I take away the tens: 78 − 30 = 48.

I take away the ones: 48 − 4 = 44.

78 − 34 = 44.

Check: 44 + 34 = 78? Tens 40 + 30 = 70, ones 4 + 4 = 8. In all 78. It matches.

53 − 7 = ?

Three is not enough for seven. Stop at ten.

To a full ten: 53 − 3 = 50.

I split the seven: 7 = 3 + 4. 4 are left to take away.

From ten: 50 − 4 = 46.

53 − 7 = 46.

Check: 46 + 7 = 53? 46 + 4 = 50, 50 + 3 = 53. Yes.

81 − 45 = ?

I take away the tens: 81 − 40 = 41.

I take away the ones: 41 − 5. One is not enough, I go through ten: 41 − 1 = 40, then 40 − 4 = 36.

81 − 45 = 36.

Check: 36 + 45 = 81? Tens 30 + 40 = 70. Ones 6 + 5 = 11. In all 70 + 11 = 81. It matches beautifully.

Practice: now you do it 🎲

Now try it for real. You will see all the questions at once, like on paper. スタート with ten. If it goes well, add more.

Subtract to 100. Tens first, then ones.

Practise the questions

On paper

Paper, a pencil and coins for checking if you like.

1. Calculate the easy cases: 67 − 25, 89 − 43.

2. Calculate the tricky cases through ten: 52 − 6, 74 − 38.

3. Check each result by adding and write the check down.

4. Make up a question about pocket money (how much is left after shopping) and solve it.

Now you 💪

1. I take away in parts: tens first, then ones.
2. When the ones are not enough, I go through a full ten and I do not flip the ones.
3. I check every difference by adding.

What to take from this lesson

• Subtract in parts: tens, then ones.
• Not enough ones? A stop at a full ten.
• Never flip the ones: 42 − 5 is not “5 − 2”.
• Check: difference + taken away = original number.
• Big subtracting = two small jobs one after another.

© 2026 Ing. Martin Polak / AlgoRhino · コンテンツ利用規約