Chapter 12
SUBTRACTION
Most of us find addition easier than subtraction. Subtraction, the way most people are taught in school,
is more difficult. It need not be so. You will learn some strategies in this chapter that will make
subtraction easy.
Firstly, you need to know the combinations of numbers that add to 10. You learnt those when you
learnt the speed maths method of multiplication. You don’t have to think too hard; when you multiply
by 8, what number goes in the circle below? You don’t have to calculate, you don’t have to subtract 8
from 10. You know a 2 goes in the circle from so much practice. It is automatic.
If a class is asked to subtract 9 from 56, some students will use an easy method and give an
immediate answer. Because their method is easy they will be fast and unlikely to make a mistake. The
students who use a difficult method will take longer to solve the problem and, because their method is
difficult, they are more likely to make a mistake. Remember my rule:
The easiest way to solve a problem is also the fastest, with the least chance of making a mistake.
So, what is the easy way to subtract 9? Subtract 10 and add (give back) 1.
What is the easy way to subtract 8? Subtract 10 and add (give back) 2.
What is the easy way to subtract 7? Subtract 10 and add (give back) 3.
What is the easy way to subtract 6? Subtract 10 and add (give back) 4.
What is the easy way to subtract 90? Subtract 100 and add (give back) 10.
What is the easy way to subtract 80? Subtract 100 and add (give back) 20.
What is the easy way to subtract 70? Subtract 100 and add (give back) 30.
What is the easy way to subtract 95? Subtract 100 and add (give back) 5.
What is the easy way to subtract 85? Subtract 100 and add (give back) 15.
What is the easy way to subtract 75? Subtract 100 and add (give back) 25.
What is the easy way to subtract 68? Subtract 70 and add (give back) 2.
Do you get the idea? The simple way to subtract is to round off and then adjust.
What is 284 minus 68? Let’s subtract 70 from 284 and add 2.
284 − 70 = 214
214 + 2 = 216
This is easily done in your head. Calculating with a pencil and paper involves carrying and
borrowing. This way is much easier.
How would you calculate 537 minus 298? Again, most people would use pen and paper. The easy
way is to subtract 300 and give back 2.
537 − 300 = 237
237 + 2 = 239
Using a written calculation the way people are taught in school would mean carrying and borrowing
twice.
To subtract 87 from a number, take 100 and add 13 (because 100 is 13 more than you wanted to
subtract).