Subtracting Reverses

There are two forms of this trick.
1. 3-digit reverses(753 - 357)
Subtract the 1st digit of the 1st number and the 1st digit of the 2nd number
7 - 3 = 4
Then plug your answer into the equation: 100n - n to get the final answer.
100(4) - 4 = 396
753 - 357 = 396
2. 4-digit reverse pairs(7568 - 6875)
Subtract the 1st pair from the second pair.
75 - 68 = 7
Then plug your answer into the equation: 100n-n to get the final answer.
100(7) - 7 = 693
7568 - 6875 = 693