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We can multiply or divide a whole number by 10, 100 or 1000

Year 5 Unit 8

What we are learning:

  • When we multiply by 10 it is tempting to say ‘you just add a zero’, but this does not promote understanding of the mathematics. Your child needs to know what is happening to the size of the numbers. Also, adding a zero will only work when multiplying whole numbers, it does not work for decimals i.e. 2.5 x 10 is 25 not 2.50.
  • Similarly, you may have heard the phrase ‘move the decimal point’, however, the decimal point does not move, the digits (numbers) do.
  • When multiplying by 10, 100 or 1000, etc, the digits move to the left, as the number is getting larger.
  • When dividing by 10, 100 or 1000, etc, the digits move to the right, as the number is getting smaller.
  • When multiplying or dividing by 10, move the digits one place in the relevant direction, when multiplying or dividing by 100, move the digits two places in the relevant direction (because 10 x 10 = 100). Look at how many zeros to tell you how many places to move the digits.
  • Use a place value chart and digit cards to model what happens to the digits.
  • Your child also needs to understand that we use zero as a ‘place holder’ e.g. if we multiply 6.3 by 100, all of the digits move two places to the left, however, we need to insert a zero into the units column to show that the answer is 630 and not 63. 

  • Children can get confused by which way the digits ‘move’. Encouraging them to think about what is happening to the size of the number will help with this.
ACTIVITY 2: DIVIDING BY 10, 100, 1000

Activities you can do at home:

Write down a list of numbers such as:
0.76    7.6    760    76000    76    0.76    0.076
Ask your child to explain how to get from one number to the next e.g. to get from 0.76 to 7.6, you multiply by 10.

Good questions to ask:

Has the number got smaller or larger? By how much?
What does that tell you about the calculation that was done to get from one number to the next?
Can you write down a list of numbers in the same way?
How do we multiply a number by 10?
How do we divide a number by 100?

If your child:

Is not sure which columns to put digits in when multiplying.
Keep using a table or grid with the columns labelled until they are confident.
Also talk about the ‘reasonableness’ of an answer, e.g. if we multiply 0.98 by 1000 we know that 0.98 is nearly 1 so the answer will be nearly 1000.
Therefore if we move the digits incorrectly and get 9.8 (instead of 980) we can see that it cannot be right.

Extension Activity

Please use this activity when you think your child understands the unit of work. It will deepen and extend your child’s understanding of this unit.


Extension activity (PDF)

Extension activity answers (PDF)