Subtracting Reverses

This trick applies when subtracting two numbers whose digits are reverses of each other (e.g., 715 - 517). For a three-digit number, the shortcut is:

Result = 99 * (first digit - last digit)

A simpler way to calculate this is:

• Take the difference between the most significant and least significant digits.
• Multiply this difference by 100.
• Subtract the original difference from the result.

For example, $812 - 218$: The difference is $8 - 2 = 6$. So, $(6 \times 100) - 6 = 600 - 6 = 594$. For four-digit numbers with zeroed middle digits (e.g., 6002 - 2006), multiply the difference by 1000 instead.