# How to help your child with mental maths – Part 2

In the next part of the mental maths series, we are going to look at multiplying and dividing by 10, 100 and 1000. Your child will come across these type of questions in their arithmetic tests but it is also a really important skill to help them answer other mental maths questions and to use within grid method for multiplication.

Children tend to start this in Year 4 and Year 5 and the aim is for children to be able to do this mentally. However, a good starting point is using a place value chart.

There are some rules to remember:

• When multiplying by 10, 100 or 1000, numbers move to the left
• When dividing by 10, 100 or 1000, numbers move to the right
• The number of places each digit in the number moves is determined by the number of zeros in the number:

By 10 = one zero = one place

By 100 = two zeros = two places

By 1000 = three zeros = three places

•  The decimal point never moves (this is very different to when I was at school when the decimal point jumped the number of places – I’m showing my age now!)

Let’s see the rules in action:

Example 1: 450 x 10 – children write the number in the first row and then move each digit in the number one place to the left because they are multiplying (move left) by 10 (one zero = one place). Then Zero the Hero flies in as a place holder to stop the numbers moving back! The answer is 4500.

 TTh (Ten Thousands) Th (Thousands) H (Hundreds) T (Tens) O (Ones) 4 5 0 4 5 0 0

Example 2: 65072 ÷ 100 – children write the number in the first row and then move each digit in the number two places to the right because they are dividing (move right) by 100 (two zeros = two places). The answer is 650.72.

 TTh Ten Thousands Th Thousands H Hundreds T Tens O Ones . t tenths h hundredths 6 5 0 7 2 6 5 0 . 7 2

Example 3: 3.2 x 1000 = children write the number in the first row and then move each digit in the number three places to the left because they are multiplying (move left) by 1000 (three zeros = three places). Then Zero the Hero flies in as a place holder to stop the numbers moving back! The answer is 3200.

 TTh Ten Thousands Th Thousands H Hundreds T Tens O Ones . t tenths h hundredths 3 . 2 3 2 0 0

Once children become confident using a place value chart for these questions, they can then try answering the questions mentally by visualising the chart and columns and remembering how many places and which way to move the numbers.

Important note to remember: Children may say they just need to add a zero for multiplying by 10 or take off two zeros for dividing by 100. However, this is not taught in school as it does not work when they progress on to decimal numbers. For example: 1.2 x 1000 – if they just add 3 zeros to the number, it does not change the number – 1.2000. Whereas, moving the number 3 places to the left gives the answer of 1200.

Have a go at these with your child:

56 x 10, 340 x 10, 2.3 x 1000, 670 ÷ 10, 5600 ÷ 100, 45089 ÷ 10000.

For more information on private tutoring to help your child in Numeracy and/or Literacy, please see the Private Tutoring page.