### Tuckshop subtraction

What it does

Tuckshop subtraction is a substantial applet arranged into six options. It is designed to help learners understand various aspects of subtraction, including a key concept that is fundamental to an understanding of subtraction - decomposition.

Decomposition can be understood as follows:

Suppose you have two unopened tubes of sweets (each containing 10 sweets) and three more sweets, making 23 sweets in all.

Let's say that you wish to give 5 sweets to a friend. You will need to 'decompose' one of the tubes, making 1 tube and 13 singles.

You can now give the 5 sweets to your friend, leaving you with 1 tube and 8 singles - i.e. 18 sweets.

The Tuckshop subtraction applet builds on these ideas in the context of selling 'chews' in a sweetshop.

How to use it

When Tuckshop subtraction is first opened, you see two green rectangular areas which are used to store chews; these correspond to the tens (tubes of chews) and the units (single chews).

To the right of the screen is a selector (see opposite) which you use to select from six options arranged in a two-way table:

- from left to right are three __levels__ (1, 2 and 3);

- vertically are two __modes__ (images and symbols).

Click in any of the six regions opposite to get more information about these six options.