A number expander is a simple aid to show the many ways of writing a number using expanded notation. For example, it can show that 236 is equal to:
A number expander can also be used for showing various representations of decimals. Play with an electronic number expander to see how 3.145 can be expanded.
Number expanders are made by folding strips of paper. Children will enjoy the challenge of folding them correctly - somewhat like a fan. Print the template or let children make their own from scratch. The folding instructions are on the templates. Children write their own numbers on the number expander, one digit per blank space. Open the number expander up in many ways to see the possibilities. Every combination is correct!
Three templates ready to print are provided. They are suitable for:
These pictures show various expansions of 3 174 682.
This works in the same way as for whole numbers but a decimal point made of Blu-Tak (or similar sticky material) must be used when the words are hidden. It is removed when the words are shown.
Various expansions of 3.145 are shown below:
Ready-made number expanders can be purchased. (an order form from one Australian supplier). Note that in the decimal versions, the decimal point is actually printed on the number expander and needs to be covered with a finger to see the different expansions.
Students work in pairs and write their own decimal number on a number expander. They then write down as many expansions as they can. Expansions can be written in words and/or fractions, e.g. 0.342 is 34 hundredths + 2 thousandths OR 34/100 + 2/1000. Groups could then swap number expanders and write down as many expansions as possible in a given period of time (say 2 minutes). At the end of this time, the new expansions are checked by the original group who made the number.
Students work in pairs. Instruct them to write a number made of only 2 non-zero digits, with zeros written in all the other columns. They then swap number expanders with another group. All groups now try to interpret the number on the expander they have been given; writing the number on paper and deciding which zeros are essential, and which can be omitted. This activity is suggested because there are many special difficulties with zero, as explained throughout this resource.
Example: 0000.307000 may then be interpreted as 0.307
Students work in pairs with 2 number expanders. On the first they write any number that has non-zero digits in the ones (units) and tenths columns. Ask them to show the expansion as a number of tenths. (For example: 2.6 would be expanded to 26 tenths). Then ask them to multiply the number by 10 (260 tenths). This result should be written on the second number expander (260 tenths) which is then (after rearrangement ) seen to be 26.0, or 26. So 2.6 x 10 was found to be 26 by virtue of arithmetic of whole numbers!
Ask them to make up more examples for multiplication by 10, 100 and 1000 followed by division by powers of 10. (Note that the parallel division examples need zeros on the right of the number, so either they start with numbers like 50 or be prepared to append zeros e.g. 4.5 =4.50)