150 Speed Math for Kids
We set up the problem like this:
29 12345
We then proceed as we did for short division. We try to divide 29
into 1, which is the fi rst digit of 12,345, and of course we can’t do
it. So we join the next digit and divide 29 into 12. We fi nd that 12
is also too small—it is less than the number we are dividing by—so
we join the next digit to get 123.
Now we divide 29 into 123. Th is is where we have a problem; most
people don’t know the 29 times table, so how can they know how
many times 29 will divide into 123?
Th e method is easy. Th is is how everyone does long division, but
they don’t always explain it this way. First, we round off the number
we are dividing by. We would round off 29 to 30. We divide by 30
as we go, to estimate the answer, and then we calculate for 29.
How do we divide by 30? Th irty is 10 times 3, so we divide by 10
and by 3 to estimate each digit of the answer. So, we divide 123 by
30 to get our estimate for the fi rst digit of the answer. We divide 123
by 10 and then by 3. To roughly divide 123 by 10, we can simply
drop the fi nal digit of the number, so we drop the 3 from 123 to get
- Now divide 12 by 3 to get an answer of 4. Write 4 above the 3
of 123. Our work looks like this:
4
29 12345
Now we multiply 4 times 29 to fi nd what the remainder will be.
Four times 29 is 116. (An easy way to multiply 29 by 4 is to multiply
30 by 4 and then subtract 4.)
4 × (30 – 1) = 120 – 4 = 116
Write 116 below 123 and subtract to fi nd the remainder.
123 – 116 = 7