Write the addition or the subtraction problem vertically so you know you are lining up the place values properly. For example, if you wish to add 40.25 and 93.02, you would write the first number and then place the second number immediately beneath the first number with the decimal points aligned. Similarly, if you wish to subtract 13.12 from 86.49, you must write 86.49 first and place 13.12 immediately beneath it with the decimal points aligned.
Perform the operation by adding or subtracting the numbers. To add 40.25 and 93.02, add the hundredths column first (the 5 and the 2) to equal 7. Add the tenths column next (the 2 and the 0) to equal 2. Add the ones column next (the 0 and the 3) to equal 3. Add the tens column last (the 4 and the 9) to equal 13. Place these numbers beneath the horizontal equal sign. To subtract 13.12 from 86.49, subtract the hundredths column first (the 9 and the 2) to equal 7. Subtract the tenths column next (the 4 and the 1) to equal 3. Subtract the ones column next (the 6 and the 3) to equal 3. Subtract the tens column last (the 8 and the 1) to equal 7.
Move the decimal point down to the answer by writing a new decimal point immediately beneath the decimal points in the addends or the decimal points in the minuend and the subtrahend. Using the same examples, the addition answer is 133.27 and the subtraction answer is 73.37.