114 Speed Math for Kids
number combines with a digit in the second to equal 9. Your check
could look like this:
1 2 3 4 5 0
6 7 8 9 0 0
4 1 7 3 5 1
- 2 1 8 6 5 5
1 4 3 8 3 5 6
Note that this only works with addition. With all other mathematical
checks you must fi nd substitutes for each number. With addition you
can cross out (cast out) any digits of the numbers you are adding.
But remember, you can’t mix digits in the numbers you are adding
with digits in the answer.
Let’s look at another example.
234
671 - 855
1,760
Here the 2 in 234 can be crossed with the 7 in 671. Th e 3 in 234
combines with the 6 in 671. Th e 4 in 234 combines with a 5 in
- Th e 1 in 671 combines with the 8 in 855. All digits are cast
out except a 5 in 855. Th at means the answer must add to 5. Let’s
now check the answer:
1 + 7 + 6 + 0 = 14
1 + 4 = 5
Our calculation is correct.
Th e fi nal check looks like this:
2 3 4 0
6 7 1 0
- 8 5 5 5
1 7 6 0 5