Appendix F
PLUS AND MINUS NUMBERS
Note to Parents and Teachers
The method of multiplication taught in this book introduces positive and negative numbers to most
children. The method makes positive and negative tangible instead of an abstract idea. Positive numbers
go above when you multiply; negative numbers go below. Students become used to the idea that when
you multiply terms that are both the same you get a positive (plus) answer. If they are different (one
above and one below) you have to subtract — you get a minus answer. Even if they don’t understand it,
it still makes sense.
How do you explain positive and negative numbers? Here is how I like to do it. To me it makes sense
if you see ‘positive’ as money people owe you. That is money you have. ‘Negative’ is money you owe,
or bills that you have to pay. Three bills of $2 is 3 times −2, giving an answer of −$6. You owe $6.
Mathematically it looks like this: 3 ×2 = −6.
Now, what if someone took away those three bills for $2? That is minus 3 bills or amounts of minus
$2. That means you have $6 more than before the bills were taken away. You could write that as: −3 ×
−2 = +6.
I tell students not to worry about the concept too much. They don’t have to understand it
immediately. I tell them we will just keep using the concept and I will keep explaining it until they do
understand. It is not a race to see who can understand it first. I tell the students that understanding will
come.
You can give examples of forward speed and head wind. Forward speed is positive; head wind is
negative. Adding and subtracting positive and negative numbers is no big deal. You add positive; you
subtract negative. It is just a matter of recognising which is which.