Multiplying by 10, 100 and 1000 are fundamental ideas in arithmetic. These ideas will eventually be used in work involving negative numbers, positive numbers, decimals and percentages, so it is very important to master them early on.
Never say ‘to multiply by ten we add a nought’. This idea certainly works for whole numbers and is a nice quick explanation, but is totally false for decimals.
e.g. 3.98 x 10 is definitely not 3.980!
If children are taught to ‘add a nought’ there will be a great deal of un-learning needed later on. Bad habits are very difficult to break.
The ideas to get across are as follows:
When multiplying by 10 the number moves one place to the left.
When multiplying by 100 the number moves two places to the left.
When multiplying by 1 000 the number moves three places to the left.
When dividing by 10 the number moves one place to the right.
When dividing by 100 the number moves two places to the right.
When dividing by 1 000 the number moves three places to the right.
(Note that the decimal point remains still; it is the numbers which are shifting position around it.)
The good news is that we have recently updated our Year 4 Multiplication worksheets with plenty of pages on the above as well as 7x, 9x, 11x and 12x tables worksheets and standard written methods of multiplying.