Note to parents and teachers
This strategy is very important. If a student has mastered multiplication using the simple formula in this book, he or she has
mastered the combinations of numbers that add to 10. There are only five such combinations.
If a student has to learn the combinations of single-digit numbers that add to more than 10, there are another twenty such
combinations to learn. Using this strategy, children don’t need to learn any of them. To subtract 8 from 15, they can subtract from 10
(which gives 2), and then add the 5, for an answer of 7.
There is a far greater chance of making a mistake when subtracting from numbers in the teens
than when subtracting from 10. There is very little chance of making a mistake when subtracting
from 10; when children have been using the methods in this book the answers will be almost
automatic.Subtraction from a power of 10
There is an easy method for subtraction from a number ending in several zeros. This can be useful when
using 100 or 1,000 as reference numbers. The rule is:
Subtract the units digit from 10, then each successive digit from 9, then subtract 1 from the digit to
the left of the zeros.
For example:We can begin from the left or right.
Let’s try it from the right first. Subtract the units digit from 10.
10 − 8 = 2
This is the right-hand digit of the answer. Then take the other digits from 9.
Six from 9 is 3.
Three from 9 is 6.
One from 1 is 0.
So we have our answer: 632.
Now let’s try it from left to right.
One from 1 is 0. Three from 9 is 6. Six from 9 is 3. Eight from 10 is 2. Again we have 632.
Here is what we really did. The set problem was:We subtracted 1 from the number we were subtracting from, 1,000, to get 999. We then subtracted
368 from 999 with no numbers to carry and borrow (because no digits in the number we are subtracting
can be higher than 9). We compensated by adding the 1 back to the answer by subtracting the final digit
from 10 instead of 9.
So what we really calculated was this:
This simple method makes a lot of subtraction problems much easier.
If you had to calculate 40,000 minus 3,594, this is how you would do it:Take 1 from the left-hand digit (4) to get 3, the first digit of the answer. (We are really subtracting
3,594 from 39,999 and then adding the extra 1.)