from 63 in your head?
63 − 9 =
I am sure you got the right answer, but how did you get it? Some would take 3 from 63 to get 60, then
take another 6 to make up the 9 they have to take away, and get 54.
Some would take away 10 from 63 and get 53. Then they would add 1 back because they took away 1
too many. This would also give 54.
Some would do the problem the same way they would when using pencil and paper. This way they
have to carry and borrow in their heads. This is probably the most difficult way to solve the problem.
Remember, the easiest way to solve a problem is also the fastest, with the least chance of making a
mistake.
Most people find the easiest way to subtract 9 is to take away 10, then add 1 to the answer. The
easiest way to subtract 8 is to take away 10, then add 2 to the answer. The easiest way to subtract 7 is to
take away 10, then add 3 to the answer.
What is the easiest way to take 90 from a number? Take 100 and give back 10.
What is the easiest way to take 80 from a number? Take 100 and give back 20.
What is the easiest way to take 70 from a number? Take 100 and give back 30.
If we go back to the problem we were working on, how do we take 75 from 12,200? We can take
away 100 and give back 25. Is this easy? Let’s try it. Twelve thousand, two hundred minus 100? Twelve
thousand, one hundred. Plus 25? Twelve thousand, one hundred and twenty-five.
So back to our example. This is how the completed problem looks:
With a little practice you should be able to solve these problems entirely in your head. Practise with
the problems below.
Test yourself
Try these:
a) 98 × 145 =
b) 98 × 125 =
c) 95 × 120 =
d) 96 × 125 =
e) 98 × 146 =
f) 9 × 15 =
g) 8 × 12 =
h) 7 × 12 =
How did you go? The answers are:
a) 14,210
b) 12,250
c) 11,400
d) 12,000
e) 14,308
f) 135
g) 96
h) 84