The 99 to 100 Exchange

Counting to 100 with the Abacus

Since we only have 2 hands (with 5 fingers on each hand and are not using American Sign Language), we can’t count to 100 with our fingers using the techniques from the previous chapters. At this point, you already understand how counting with your fingers works, so let’s go directly to counting to 100 with an abacus.

To start, we will count to 9 in both the ten and ones column, by pushing 1 bead to the bar.  Then when all the beads are used below the bar, we push the 5 bead to the bar and clear the 4 beads below the bar and continue counting until we reach 9 in both columns.  To count from 99 to 100, we push 1 bead to the bar in the hundreds column and clear all the beads in the tens and ones column. This is the 99 to 100 exchange.

Adding using the 99 to 100 Exchange

As you saw in the previous section, counting to 5 involves using all the beads below the bar and the 5-bead.  Now, let’s add some single numbers and use the 99 to 100 exchange: 

• Add:  96 + 6

Examples

These examples we will use the 99 to 100 exchange in any column when no beads are available below the bar and the 5-bead is not being used in the column to its left.  Each addition will start in the left most column and continue in each column moving to the right until all the columns have been added. 

• Example:  23 + 78

• Example:  288 + 115

• Example:  763 + 239

• Example:  1,799 + 2,203

• Example:  86,556 + 13,445

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.