Decimal Operations: Add, Subtract, Multiply, Divide — 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 = 1.2 because 3 × 4 = 12.
Corrected: 0.3 × 0.4 = 0.12 because 3 × 4 = 12 and there are two total decimal places in the factors.
The corrected sentence is: "0.3 × 0.4 = 0.12 because 3 × 4 = 12 and there are two total decimal places in the factors.". The original sentence "0.3 × 0.4 = 1.2 because 3 × 4 = 12." contained an error that needed to be fixed.
2. Fix the sentence:
2.5 × 6 = 1.50 since 25 × 6 = 150 and you move the decimal one place.
Corrected: 2.5 × 6 = 15.0 since 25 × 6 = 150 and one decimal place gives 15.0.
The corrected sentence is: "2.5 × 6 = 15.0 since 25 × 6 = 150 and one decimal place gives 15.0.". The original sentence "2.5 × 6 = 1.50 since 25 × 6 = 150 and you move the decimal one place." contained an error that needed to be fixed.
3. Fix the sentence:
1.2 × 0.5 = 6.0 because 12 × 5 = 60.
Corrected: 1.2 × 0.5 = 0.60 because 12 × 5 = 60 and two decimal places gives 0.60.
The corrected sentence is: "1.2 × 0.5 = 0.60 because 12 × 5 = 60 and two decimal places gives 0.60.". The original sentence "1.2 × 0.5 = 6.0 because 12 × 5 = 60." contained an error that needed to be fixed.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. 4.3 × 7 = 30.1
4.3 has 1 decimal place. 43 × 7 = 301. Place decimal 1 place from right: 30.1.
2. 0.6 × 0.8 = 0.48
0.6 × 0.8: multiply 6 × 8 = 48. Total decimal places = 1+1=2. Place decimal 2 places from right: 0.48.
3. When multiplying 3.2 × 1.4, the product has 2 total decimal places.
3.2 has 1 decimal place and 1.4 has 1 decimal place. The product must have 1+1=2 decimal places. 32 × 14 = 448, so the product is 4.48.
4. 5.05 × 4 = 20.20
5.05 has 2 decimal places. 505 × 4 = 2020. Place decimal 2 places from right: 20.20.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. How do you decide where to put the decimal point in a multiplication answer?
Sample answer: Count the total decimal places in both factors, then place the decimal that many places from the right in the product.
The rule: add up all decimal places in the factors, then count that many digits from the right of the whole-number product to place the decimal point.
2. A ribbon costs $3.25 per meter. What is the cost of 3 meters?
Sample answer: 3.25 × 3 = 9.75, so 3 meters of ribbon costs $9.75.
3.25 × 3: 325 × 3 = 975. 2 decimal places → $9.75.