Arrange the numbers into columns -- hundreds, tens and ones.
Subtract the top number in the hundreds column from the bottom number in the hundreds column. Remember the difference.
Subtract the top number in the tens column from the bottom number in the tens column. Remember the difference.
Subtract the top number in the ones column from the bottom number in the ones column. Remember the difference.
Find the total using mental math from the partial differences found in steps 1, 2 and 3. If the smaller number is on top, you will subtract the difference in the mental math step. If the larger number is on top, you will add the difference in the mental math step. This is the answer.