In my younger, student years I remember excitedly landing a great casual job for new years day. For those youngsters who are happy not to have a big night on New Year’s Eve and happy to work on New Year’s Day, a job at the Perth Cup behind the bar is an awesome way to make some good money. The catch was, I arrived, had a quick orientation, no training and was then thrown behind the bar and the crowd of people screaming to be served was about four rows deep. Then the real clincher was that the tills were not fancy modern tills where you could add up the drinks by just clicking on the screen, instead all the adding up had to be done mentally. For those not confident in mental maths, it gets even worse… calculating the amount of change to be given to the customer had to be worked out mentally too! There was no typing in the amount of money you were given and letting the till tell you how much change to give. This is hard enough for most at the best of times but add the pressure of literally hundreds of people anxiously wanting service and this task then becomes very tricky.

The skill of counting backwards is a technique used to quickly and accurately calculate how much change is required. This was common practice before computerised cash registers began doing all the hard work for us. How would you calculate the change if the total is $13.45 and you are handed a $20 note? With counting backwards you count up from $13.45 rather than taking $13.45 away from $20. Children are not often taught this skill and without it many kids and adults would try to visualise a mental notepad trying to calculate in their head the same way we would calculate it on paper, borrowing and carrying. The problem with this is that our visual memory can only hold an image for a mere few seconds. So once you have borrowed from the 2 and worked it across to the right the numbers are beginning to disappear. It is very hard to calculate like this in your head especially under pressure. In my Perth cup bar experience, I had to very quickly master the art of counting backwards. It is a very handy skill to have up your sleeve, even if you’re not planning a stressful job behind the counter.

Steps for counting backwards to calculate change quickly and accurately

How much change is needed for $13.45 out of $20?

Step 1: Say the purchase price, in this case $13.45.

Step 2: Starting with the cents count up to the nearest dollar. Start with the smallest demonination. Using the 45c as a starting point. 5c gets you to 50c and then another 50c gets you to the dollar. Therefore counting the 55c onto the $13.45 brings you up to $14.00.

Step 3: Now we want to count up to the nearest $5. So from $14.00 we only need to add $1.00 and we are up to $15.00.

Step 4: Now we can use lots of 5s or 10s to get us up to the total. In this case $5.00 is added to the $15.00 to reach the $20.00 we started with.

So in summary, in counting backwards, we would say, $13.45, hand over 5c and say $13.50, then hand over 50c and say $14.00, then hand over $1 and say $15.00 and then a $5 note and say $20.00. That is a total of $6.55 change.

Even if you are not planning on working the tills for excellent pay on New Year’s Day (and if the tills are going to do all the calculating for you) this skill is still extremely useful for you and your children to be able to check the change that they are given. People make mistakes (and sometimes even try to trick you) so being able to quickly calculate the change yourself is a great skill for life.

 Photo credit: QuiteLucid / Foter / CC BY-NC-SA