The 100 to 99 Exchange

Subtracting with the 100 to 99 Exchange

The 100 to 99 exchange involves using the current column and the two columns to its left. You will need to use this exchange when you subtract 1 in the current column when the current column and the column to its left have no beads are available. This means if you are subtracting 1 from 100, 200, 300 … 900, then you will need to use this exchange.  While this exchange may seem unnecessary, many students have trouble with this situation.  

This is how this exchange works:

• Subtract:  104 – 6   

Examples

In these examples we will use the 100 to 99 exchange to subtract larger numbers. Each subtraction will start in the left most column of the first number and continue in each column moving to the right until all the columns have been subtracted. 

• Example:   422 – 225  

• Example:  325 – 127 

• Example:  276 – 78 

• Example:  931 – 334 

• Example:  2,658 – 659  

Build Your Skill

Now you are ready to try some calculations on your own. Click on this link for some problems and their solutions to test your skill.