Here
is a picture that I saw the other day. Many of you may have seen this already
as it appears to be circulating on various social media websites.
At
first glance, I was thinking this is ridiculous. What in the world is this? Are
we supposed to teach math like this? What do all these numbers really mean? It appears
to be a method of doing subtraction that takes multiple steps of addition that
seem to make no sense at all. I read this article on the topic and it made much
more sense to me.
The
upper portion is how many of us learned to do subtraction questions. Often times,
these numbers that we determine are based on what the calculator tells you,
what you calculate with your fingers, or what you have memorized over the
years.
The
bottom portion is something many of us actually do in our heads. When I look at
the question 32 - 12, yes I know the answer is 20 (from memorization) but if I needed
to calculate it in my head, I would think 12 to 22 is 10, 22 to 32 is also 10. So
now, 10 and 10 make 20. This is also how many of us calculate how much change
we should get back when we pay for an item or service. We calculate how many
cents to get to the next dollar, and then just add up how many more dollars we
need.
Example:
You
owe $4.90, and you hand the cashier a $20.
$20
– $4.90
To
perform the mental math, I would think $0.10 to get to $5, and then $15 to get
to $20. Thus, I would get $15.10 in change back.
Essentially, that is what the bottom half of the original picture is depicting. The article also had a second image, which contains an easier description of how this method works.
I think this is a great way to teach subtraction, but I do not think it should be the only method that is used. Students need to be given the option of which method suits their understanding the most and be allowed to use it accordingly.
