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:
In Grade 4, to compute 4.0 minus 2.35 you simply subtracts 2.35 from 4 and gets 2.35 as the answer.
Corrected: In Grade 4, to compute 4.0 minus 2.35 you rewrite 4.0 as 4.00, regroup, and get 1.65 as the answer.
Subtraction requires equal decimal places to regroup correctly.
2. Fix the sentence:
A Grade 4 learner thought 7.2 minus 3.45 equal 3.85 because she ignore the alignment of decimal places.
Corrected: A Grade 4 learner found 7.2 minus 3.45 equals 3.75 because she rewrote 7.2 as 7.20 and regrouped carefully.
Regrouping borrows one tenth as ten hundredths.
3. Fix the sentence:
When subtracting 5.0 from a Grade 4 problem like 5.0 minus 1.78, you gets 4.22 by skipping the zeros.
Corrected: When subtracting in a Grade 4 problem like 5.0 minus 1.78, you get 3.22 by writing 5.00 and regrouping twice.
You must regroup across multiple decimal places.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. Grade 4: To find 4.0 - 2.35, first rewrite 4.0 as 4.00 before subtracting.
Equal decimal places allow column subtraction.
2. Grade 4: The difference 8.0 - 3.47 equals 4.53 after annexing zeros and regrouping.
Regrouping moves one tenth into ten hundredths.
3. Grade 4: When subtracting 6.5 - 2.78, regroup by changing one tenth into ten hundredths.
One tenth equals ten hundredths.
4. Grade 4: 9.0 minus 5.62 equals 3.38 when you annex zeros and subtract carefully.
Regrouping across ones, tenths, and hundredths gives 3.38.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. Grade 4: Explain why you must annex zeros when subtracting 4.0 - 2.35.
Sample answer: You annex zeros so 4.0 becomes 4.00, giving the minuend hundredths place to subtract from. Without zeros, you cannot regroup tenths and hundredths properly. After regrouping, 4.00 - 2.35 = 1.65.
Place value alignment is needed for borrowing.
2. Grade 4: Describe how to regroup when computing 7.0 - 3.48.
Sample answer: Rewrite 7.0 as 7.00. To subtract 8 hundredths from 0 hundredths, borrow one tenth as ten hundredths, then borrow one whole as ten tenths. Subtract column by column to get 3.52.
Each regrouping converts one unit into ten of the next smaller place.