Students correct three subtraction errors — wrong LCD, subtracting both numerators and denominators, and skipping the conversion step. Part B has four fill-in-the-blank questions about the LCD of sixths and fourths, a 3/4 − 1/6 result, and converting 2/3 to fifteenths. Part C has two short-answer questions about forgetting the LCD and computing 5/6 − 1/9.
Subtraction requires the same LCD process as addition, and the same errors appear — correcting both operations together reinforces the full unlike-denominator procedure.
Style:
Adding and Subtracting Fractions with Unlike Denominators
Part A: Fix the Sentence
Each sentence has an error. Rewrite it correctly on the line.
1. Fix the sentence:
To subtract 34 − 13, the LCD is 8 so the answer is 68 − 28 = 48.
Rewrite: To subtract 34 − 13, the LCD is 12 so the answer is 912 − 412 = 512.
2. Fix the sentence:
56 − 12 equals 44 because you subtract both the numerators and denominators.
Rewrite: 56 − 12 equals 56 − 36 = 26, which simplifies to 13.
3. Fix the sentence:
The difference of 78 − 14 is 58 because you subtract 1 from 7 and keep the 8.
Rewrite: The difference of 78 − 14 is 78 − 28 = 58 after converting 14 to 28.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. The LCD of 56 and 14 is 12.
2. 34 − 16 = 912 − 212 = 712.
3. To subtract 23 − 15, rewrite 23 as 10 15 .
4. 710 − 15 = 710 − 210 = 12 in simplest form.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. What happens if you forget to find the LCD before subtracting fractions?
You get a wrong answer because you are subtracting parts of different sizes.
2. What is 56 − 19? Show your final answer in simplest form.
The answer is 1318 because 1518 − 218 = 1318.
Adding and Subtracting Fractions with Unlike Denominators
★ Part A: Fix the Sentence
Each sentence has an error. Rewrite it correctly on the line.
1) Fix the sentence:
To subtract 3/4 − 1/3, the LCD is 8 so the answer is 6/8 − 2/8 = 4/8.
Rewrite: To subtract 3/4 − 1/3, the LCD is 12 so the answer is 9/12 − 4/12 = 5/12.
2) Fix the sentence:
5/6 − 1/2 equals 4/4 because you subtract both the numerators and denominators.
Rewrite: 5/6 − 1/2 equals 5/6 − 3/6 = 2/6, which simplifies to 1/3.
3) Fix the sentence:
The difference of 7/8 − 1/4 is 5/8 because you subtract 1 from 7 and keep the 8.
Rewrite: The difference of 7/8 − 1/4 is 7/8 − 2/8 = 5/8 after converting 1/4 to 2/8.
★ Part B: Fill in the Blank
Write the missing word or number on each line.
1) The LCD of 5/6 and 1/4 is 12.
2) 3/4 − 1/6 = 9/12 − 2/12 = 7/12.
3) To subtract 2/3 − 1/5, rewrite 2/3 as 10/15.
4) 7/10 − 1/5 = 7/10 − 2/10 = 1/2 in simplest form.
★ Part C: Short Answer
Answer each question in one or two complete sentences.
1) What happens if you forget to find the LCD before subtracting fractions?
You get a wrong answer because you are subtracting parts of different sizes.
2) What is 5/6 − 1/9? Show your final answer in simplest form.
The answer is 13/18 because 15/18 − 2/18 = 13/18.
Ready to Practice?
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9 Questions
15-20 minutes
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