Subtraction 125
Th ree 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. Th ree from 9 is 6. Six from 9 is 3. Eight from 10
is 2. Again we have 632.
Here is what we really did. Th e set problem was:
1 0 0 0
- 3 6 8
6 3 2
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 fi nal digit from 10 instead of 9.
So what we really calculated was this:
9 9 9 + 1 - 3 6 8
6 3 1 + 1
Th is 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:
4 0 0 0 0 - 0 3 5 9 4
Take 1 from the left-hand digit (4) to get 3, the fi rst digit of the answer.
(We are really subtracting 3,594 from 39,999 and then adding the
extra 1.)