Subtracting Reverses
What's a reverse? A reverse is a number written backwards from
another number. So, 256 is the reverse of 652. There is a neat
little trick dealing with the subtraction of reverses.
1 Method with Nines
When you subtract two three-digit reverses, a pattern emerges
with the difference. The difference is usually a three-digit
answer, except in one case, where it is the number 99.
- The first digit of the difference is the difference in
the hundreds places, minus 1.
- The last digit is the number you need to add to the first
digit of the difference to get 9.
Example:
764 - 467 =
The first digit is the difference in the hundreds place, minus
1: 7 - 4 - 1 = 2.
The center digit is 9.
The last digit is the number you add to the first digit to get
9, or 9 - 2 = 7.
Therefore, 764 - 467 = 297.
Example
423 - 324 =
The first digit is the difference in the hundreds place, minus
1: 4 - 3 - 1 = 0. Since it's 0, you don't write anything.
The center digit is 9.
The last digit is 9 - 0 = 9.
Thus, 423 - 324 = 99.
Also, watch out for problems where you are subtracting a
smaller number minus a larger number. The process is the same,
using the bigger number as your starting point. Don't forget to
put a negative sign on the answer.
Example:
357 - 753 =
You have to think of this one backwards. You still subtract
the hundreds places, minus 1: 7 - 3 - 1 = 3.
The center number is 9.
9 - 3 = 6.
Therefore, 357 - 753 = -396. Don't forget the negative. | 
