Subtracting numbers using the column and chunking method

Speaker: Subtraction is what you do when you take one amount away from another.

It's calculating the difference between two numbers.

Elevator Speaker: Going down.

Speaker: I start on the 7th floor and take the lift down to the 2nd.

To work out how many floors I'm going down, I need to subtract the floor I want to end up on from the floor I started on. So I've gone down 5 floors.

Speaker: I've started with 12 boxes and dropped off 2.

So now I've got 10 left.

I've then got to drop off another 4.

So that's 10 minus 4, which leaves me with 6.

No deliveries means I've got nothing, or 0, to take away.

Subtracting 0 means I've still got 6 left.

Speaker: I need to work out how many glasses are being returned.

The boxes sometimes have glasses missing.

The easiest way is to count the empty spaces. 7 missing, 13 left.

But if you don't have the boxes in front of you, you've got to work it out.

When the boxes are full they have 20 in each box. So 20 minus 13, 3 from 0 I can't do.

So I exchange one of the tens for 10 units and put it in the units column.

1 from 1 in the tens column is 0, so 7 missing in total.

If these 2 boxes were full there would be 40 glasses altogether. But I've only got 29.

To make it easier I'm going to break down the 29 into 20 and 9.

First I take away the 20 from the 40 and then the 9, leaving me with 11.