Multiplying Above & Below the Reference Number 31
A shortcut for subtraction
Let’s take a break from this problem for a moment to have a look at
a shortcut for the subtractions we are doing.
What is the easiest way to subtract 75? Let me ask another question.
What is the easiest way to take 9 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. Th en they would add
1 back because they took away 1 too many. Th is would also give 54.
Some would do the problem the same way they would when using
pencil and paper. Th is way they have to carry and borrow in their
heads. Th is is probably the most diffi cult 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 fi nd the easiest way to subtract 9 is to take away 10,
then add 1 to the answer. Th e easiest way to subtract 8 is to take
away 10, then add 2 to the answer. Th e 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
brent
(Brent)
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