# Subtraction by chunking

What it does

Subtraction by chunking shows a modern method of subtraction based on the idea of chunking. This involves dealing with the tens and units separately (even if this produces a minus number) and then putting the two numbers together at the end.

How to use it

Set the option 'numbers chosen at random' from the drop-down menu at the top.

1. In the example opposite (93 - 79), first subtract the tens (90 - 70), which gives 20. Click the 20 in the grid at the bottom of the screen and then click on the 'subtract tens' box.

2. Subtracting the units gives 3 - 9 (i.e. -6). In the grid, click on the minus sign followed by the 6 and then click on the 'subtract units' box.

3. Finally, add the two numbers 20 and -6, giving 14. Click the 10 followed by the 4 and then click on the '=' box.

All the boxes should turn blue, indicating that the subtraction has been successfully completed. If an incorrect number is entered into any of the boxes, instead of turning blue, an 'X' appears next to that box. To correct it, simply insert the correct number using the method described above.

**Screenshot of Subtraction by chunking**