Young Einstein Mathematics Educational Software Contents

Activity Number 20

Heading : Subtracting 3 Digit Numbers

Description : Subtracting a 3 digit number

Aim/Questions : Perform the subtraction.

Educational software theory used:

Subtracting 3 Digit Numbers

You need to understand the borrow concept first. Lets look at an example.

The number 62 =

Tens              Ones

XXXXXX

XXXXXX

XXXXXX

XXXXXX

XXXXXX

XXXXXX

XXXXXX

XXXXXX

XXXXXX     X

XXXXXX     X

The number 62 is 6 groups of ten plus 2 groups of one.

But 62 can also be 5 groups of ten and 12 groups of one.

Tens              Ones

XXXXX      X

XXXXX      X

XXXXX      X

XXXXX      X

XXXXX      X

XXXXX      X

XXXXX      X

XXXXX      X

XXXXX      X

XXXXX      X

           X

           X

All we did in the second case was borrow one group of tens and put them in the ones column. You will see how this makes our subtraction easier.

When subtracting larger numbers start with the ones units first.

When you have completed the ones units then move to the next column of tens units and subtract the tens units.

Then finally move to the hundreds units.

Eg

 62 - 15

Firstly work on the ones units

 62 – 15 =   ?? I can’t do this.

Lets change 62 from 6 tens and 2 ones to 5 tens and 12 ones and try again.

512 - 15 =  7

Now lets work on the tens units. But remember that you borrowed one lot of tens and you have only 5 left.

52  - 15 = 47

Therefore 62 – 15 = 47

Lets look at a larger example

 642 – 356

 642 – 356 doesn’t work

6312 - 356    so borrow and make it 12 - 6 = 6

Now for the tens units leaving the answer in the tens column

 642 - 356 was changed above to 3-5 because of the borrow

 635 - 356 doesn’t work

5135 - 356 so borrow from the hundreds units this time

                      and make it 13 – 5 =  86

Finally the hundreds units

642  - 356 was changed above to 5-3 because of the borrow

542  – 356 = 286

Therefore 642 – 356 = 286

Educational software Instruction :

1.  Subtract the number then type the difference into the ‘Answer Box’ provided.

2.  Press ‘Enter’ when you have finished.

Click on the ‘Wording’ check box if you want the sum written in a sentence.

It might be easier to have backwards entry enabled for this exercise.

Module 20. These Instructions from 'Young Einstein Mathematics educational software by Maths Practice are Copyright. Copyright Maths Practice P/L 2003 ©