Rounding and Decimals:
Addition and Subtraction with Decimal Numbers
Addition and subtraction with decimals are just the same as addition and subtraction with the whole number. The most important thing to remember is to line up the decimal points so that when you stack them, they are directly on top of each other. Doing this stacks the place values so that the ones are on top of the ones, the tens on the tens, the tenths on the tenths, and so forth. If there are any times when there aren’t as many digits after the decimal place in one number as there are in another, remember that you can fill in any places to the right of the decimal with a 0. In numbers, this looks like 3.15 = 3.150 = 3.150000000 and so on.
Steps for Addition (Combining Two Positive or Two Negative Numbers)
- Stack the numbers according to place value. (This will line up the decimals.)
- Add the numbers in the columns starting with the column on the right.
- Carry numbers into the next place value to the left as needed.
- Repeat this pattern for each remaining column.
- Bring the decimal down.
- Check sign. (If both numbers were positive, the answer is positive. If both numbers were negative the answer is negative.)
Steps for Subtraction (Combining a Positive and Negative Number)
- Biggest on top.
- Stack in columns according to place value. (This will line up the decimals.)
- Regroup when needed.
- Subtract in columns by place value starting on the right and going left.
- Bring the decimal down.
- Strongest wins. (If the bigger number was negative then the answer is negative. If the bigger number was positive then the answer is positive.)
Video Source (06:14 mins) | Transcript
Video Source (08:49 mins) | Transcript
Additional Resources
- Khan Academy: Adding decimals (02:40 mins)
- Khan Academy: Subtracting decimals (2:08 mins)
Practice Problems
Evaluate the following expression:
- 2.4 + 8.8 = ?
- 8.12 + 9.81 = ?
- 5.92 + 3.22 = ?
- \(7.2-1.5=?\)
- \(5.2-5.4=?\)
- \(39.6-88.88=?\)
