Myriora Maths › Maths foundations › Chapter 3: Taking away

Clever subtraction

Ages 7–9Chapter 3 · Lesson 5

In this lesson

When a number is close to a round number, take away the round number instead.
If you took away too many, add the extra back.
Or add the same amount to both numbers: the gap stays the same.

Your turn

What is six hundred and twenty-five, minus three hundred and ninety-seven?

Show the answer

Three hundred and ninety-seven is three less than four hundred. Six hundred and twenty-five minus four hundred is two hundred and twenty-five. Give back the three: two hundred and twenty-eight!

The common mistake

Here's a question that catches lots of people. What is five hundred and three minus one hundred and ninety-eight? Some people take away two hundred, then take away two more, and get three hundred and one. But taking away two hundred already took away too much! You must give the two back: three hundred and five.

Try it at home

Prices like £1.99 or 98p: take away the round price, then add the bit back.

Read the whole lesson

Hello, and welcome to Myriora Maths! What is five hundred and three, minus one hundred and ninety-eight? Column subtraction would need lots of exchanging. There's a much cleverer way!

Look at one hundred and ninety-eight. It's very close to two hundred: just two less. And taking away two hundred is easy!

So take away two hundred: five hundred and three minus two hundred is three hundred and three. But we only needed to take away one hundred and ninety-eight. We took away two too many! So give the two back: three hundred and three plus two is three hundred and five.

Why do we add the two back? Look at the number line, starting at five hundred and three. Taking away two hundred jumps back to three hundred and three. That's two further than we wanted to go. So step forward two, to three hundred and five.

Here's another clever way. Add two to both numbers: five hundred and five, minus two hundred. The gap between the two numbers stays the same, so the answer stays the same. Five hundred and five minus two hundred is three hundred and five.

Let's try one together. What is seventy-four minus twenty-nine? Twenty-nine is nearly thirty. Say it out loud!

Seventy-four minus thirty is forty-four. But we took away one too many. So give one back: forty-five!

Your turn! What is six hundred and twenty-five, minus three hundred and ninety-seven? Pick the right card. You've got fifteen seconds, or pause the video. Go!

Time's up! Three hundred and ninety-seven is three less than four hundred. Six hundred and twenty-five minus four hundred is two hundred and twenty-five. Give back the three: two hundred and twenty-eight!

Quick review. When a number is close to a round number, take away the round number instead. If you took away too many, add the extra back. Or add the same amount to both numbers: the gap stays the same. See you next time!