Speed Math for Kids Achieve Their Full Potential

Three from 9 is 6. Five from 9 is 4. Nine from 9 is 0. Four from 10 is 6.
The answer is 36,406.
You could do the calculation off the top of your head. You would call out the answer, ‘Thirty-six
thousand, four hundred and six.’ Try it. With a little practice you can call out the digits without a pause.
You may find it easier just to say, ‘Three, six, four, oh, six.’ Either way it is very impressive.

Test yourself

Try these   for yourself:

a)  10,000  −   2,345

b)  60,000  −   41,726

You would   have    found   the answers are:

a)  7,655

b)  18,274

Subtracting smaller numbers

If the number you are subtracting is short, then add zeroes before the number you are subtracting (at
least mentally) to make the calculation.
Let’s try 45,000 − 23:

You extend the zeroes in front of the subtrahend (the number being subtracted) as far as the first digit
that is not a 0 in the top number. You then subtract 1 from this digit. Five minus 1 equals 4.
Subtract each successive digit from 9 until you reach the final digit, which you subtract from 10.
This is useful for calculating the numbers to write in the circles when you are using 100 or 1,000 as
reference numbers for multiplication. It is also useful for calculating change.
The method taught in Australian and North American schools has you doing exactly the same
calculation, but you have to work out what you are carrying and borrowing with each step. The benefit
of this method is that it becomes mechanical and can be carried out with less chance of making a
mistake.

Checking subtraction by casting nines

For subtraction, the method used to check answers is similar to that used for addition, but with a small
difference. Let us try an example.

Is  the answer  correct?    Let’s   cast    out the nines   and see.

Five minus 8 equals 6? Can that be right? Obviously not. Although in the actual problem we are
subtracting a smaller number from a larger number, with the substitutes, the number we are subtracting
is larger.
We have two options. One is to add 9 to the number we are subtracting from. Five plus 9 equals 14.
Then the problem reads:

14  −   8   =   6