 In the last post, we looked at multiplying and dividing by 10, 100 and 1000. We are now going to look at how children can apply this skill to help them with other mental maths questions which will arise in the arithmetic test and during grid method for multiplication.

Multiplying a two digit number by a one digit number

Example 1: 34 x 5

Children can partition the number into tens and ones:             34 = 30 and 4

Then multiply each of these numbers by 5:                                   30 x 5 = 150 and 4 x 5 = 20

If children struggle with the 30 x 5 part, they can divide 30 ÷ 10 (moving numbers one place to the right) to allow them to answer 3 x 5 = 15, which they know from their tables. Then, they need to multiply the answer by 10 (moving numbers one place to the left): 15 x 10 = 150.

Example 2: 63 x 4

Children can partition the number into tens and ones:             63 = 60 and 3

Then multiply each of these numbers by 4:                                   60 x 4 = 240 and 3 x 4 = 12

If children struggle with the 60 x 4 part, they can divide 60 ÷ 10 (moving numbers one place to the right) to allow them to answer 6 x 4 = 24, which they know from their tables. Then, they need to multiply the answer by 10 (moving numbers one place to the left): 24 x 10 = 240.

Using known facts to multiply and divide

This is a Year 5 and 6 skill which depends on children being confident with their times tables. For more help with tables, see How to help your child learn their times tables and How to make times table learning fun!

Once children are confident with their times tables, they can use their times table facts to work with multiples of 10, 100 and 1000.

The general idea to make it easier to answer the question is to get back to the original times table by dividing each number (if needed, it may only be one of the numbers) by 10, 100 or 1000 (depending on the number of zeros in the number). Then, multiply the answer by the same amount.

e.g. Using their knowledge of 6 x 7 = 42 and their skills of multiplying and dividing by 10, 100 and 1000, they can try:

Now try these with your child: 7 x 8 , 70 x 8, 7 x 800, 70 x 80, 70 x 800, 560 ÷ 8, 5600 ÷ 70, 560 ÷ 70.

Grid method

When you are confident with multiplying and dividing by 10, 100 and 1000, it will be really useful for grid method. For further information, take a look at my post on multiplication written methods, including grid method.

Below is an example of grid method. The final answer is found by adding together the red amounts.

 x 3000 200 70 6 3 9000 600 210 18

Again, the general idea to make it easier to answer the question is to get back to the original times table by dividing each number (if needed, it may only be one of the numbers) by 10, 100 or 1000 (depending on the number of zeros in the number). Then, multiply the answer by the same amount.

If your child finds 3000 x 3 tricky, they can do 3000 ÷ 1000 = 3, which allows them to answer 3 x 3 = 9. Then, they need to multiply the answer by 1000 so 9 x 1000 = 9000.

If your child finds 200 x 3 hard to answer, they can do 200 ÷ 100 = 2, which allows them to answer 2 x 3 = 6. Then, they need to multiply the answer by 100 so 6 x 100 = 600.

To help with 70 x 3, they can do 70 ÷ 10 = 7, which allows them to answer 7 x 3 = 21. Then, they need to multiply the answer by 10 so 21 x 10 = 210.