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 center digit is 9.
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.
