124 Speed Math for Kids
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
Th ere is an easy method for subtraction from a number ending in
several zeros. Th is can be useful when using 100 or 1,000 as reference
numbers. Th e 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:
1 0 0 0
- 3 6 8
We can begin from the left or right.
Let’s try it from the right fi rst. Subtract the units digit from 10.
10 – 8 = 2
Th is is the right-hand digit of the answer. Th en take the other digits
from 9.
Six from 9 is 3.