Adding and Subtracting Decimals
Apply knowledge of place value to add and subtract decimals; use estimation and rounding to check the reasonableness of answers.
Read this first
Line up the decimal points. Then it is ordinary column work.
Estimate first. 3.4 plus 2.8 is about 6.
Words you need
- Place value
- What a digit is worth, based on its column.
- Estimate
- A quick close answer.
- Reasonable
- An answer that makes sense.
Worked example
e.g. What is 3.4 + 2.85?
- Estimate first. About 3 plus 3 is 6.
- Write the numbers with the points lined up.
- Add a zero so both have two decimal places.
- Add each column. 3.40 + 2.85 = 6.25.
- 6.25 is close to 6. The answer is reasonable.
Never right-align the digits. Align the points.
Practice
The pictures fade as you go. By the last few you are on your own.
1What is 5.6 + 1.3?with a picture
Answer6.9.
2What is 8.2 - 3.7?with a picture
Answer4.5.
3What is 4.75 + 2.5?with a hint
Need a hint?
Write 2.5 as 2.50 first.
Answer7.25.
4What is 10 - 2.4?with a hint
Need a hint?
Write 10 as 10.0.
Answer7.6.
5What is 12.05 + 3.7?on your own
Answer15.75.
6A bill is 50. What is the change?on your own
Answer$26.60.
7Why estimate before adding decimals?on your own
AnswerTo catch a misplaced decimal point.
Think about it
1
Why add a zero to the end?
One good answerIt fills the empty column so every column has a digit.
2
What goes wrong if you line up the last digits?
One good answerThe tenths get added to the hundredths.
Common mistakes
Right-aligning the digits.
Line up the points instead.
Forgetting the point in the answer.
It sits straight below the others.