Decimal Addition and Subtraction — Answer Key
Part A: Fix the Sentence
Each sentence has an error. Rewrite it correctly on the line.
1. Fix the sentence:
0.3 + 0.4 = 0.34
Corrected: 0.3 + 0.4 = 0.7
Grade 4 students align decimal points so 3 tenths plus 4 tenths equals 7 tenths, written as 0.7.
2. Fix the sentence:
0.8 - 0.5 = 0.5
Corrected: 0.8 - 0.5 = 0.3
Grade 4 learners stack the decimals and subtract 5 tenths from 8 tenths to get 3 tenths, or 0.3.
3. Fix the sentence:
0.6 + 0.2 = 0.62
Corrected: 0.6 + 0.2 = 0.8
Grade 4 students recognize that 6 tenths plus 2 tenths equals 8 tenths, written 0.8, never concatenated as 0.62.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. Add the tenths: 0.5 + 0.4 = 0.9
Grade 4 students align decimals and add tenths: 5 + 4 = 9 tenths, so the sum is 0.9.
2. Subtract: 0.7 - 0.3 = 0.4
Grade 4 learners subtract tenths after aligning the decimal points, giving 4 tenths or 0.4.
3. 0.2 + 0.6 = 0.8
Grade 4 students add 2 tenths and 6 tenths to get 8 tenths, written 0.8.
4. 0.9 - 0.4 = 0.5
Grade 4 learners line up decimal points and subtract 4 tenths from 9 tenths, getting 0.5.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. Why must you align the decimal points before adding 0.3 + 0.4?
Sample answer: Aligning decimal points keeps tenths under tenths and ones under ones. For 0.3 + 0.4, lining them up shows 3 tenths plus 4 tenths equals 7 tenths, so the answer is 0.7.
Grade 4 students must align decimal points so each digit is added to the digit with matching place value, giving the correct sum.
2. Explain how 0.8 - 0.5 = 0.3 using tenths.
Sample answer: The number 0.8 means 8 tenths and 0.5 means 5 tenths. Subtracting 5 tenths from 8 tenths leaves 3 tenths, which is written as 0.3.
Grade 4 learners use place value language to explain decimal subtraction, connecting 0.8 to 8 tenths and 0.5 to 5 tenths.