Lesson 21
Using Operations on Decimals to Solve Problems
Problem 1
A roll of ribbon was 12 meters long. Diego cut 9 pieces of ribbon that were 0.4 meter each to tie some presents. He then used the remaining ribbon to make some wreaths. Each wreath required 0.6 meter. For each question, explain your reasoning.

How many meters of ribbon were available for making wreaths?

How many wreaths could Diego make with the available ribbon?
Solution
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Problem 2
The Amazon rainforest covered 6.42 million square kilometers in 1994. In 2014, it covered only \(\frac{50}{59}\) as much. Which is closest to the area of the Amazon forest in 2014? Explain how you know without calculating the exact area.
6.4 million km^{2}
5.4 million km^{2}
4.4 million km^{2}
3.4 million km^{2}
2.4 million km^{2}
Solution
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Problem 3
To get an A in her math class, Jada needs to have at least 90% of the total number of points possible. The table shows Jada’s results before the final test in the class.
Jada’s points  total points possible  

Homework  141  150 
Test 1  87  100 
Test 2  81  100 
Test 3  91  100 
 Does Jada have 90% of the total possible points before the final test? Explain how you know.
 Jada thinks that if she gets at least 92 out of 100 on the final test, she will get an A in the class. Do you agree? Explain.
Solution
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Problem 4
Find the following differences. Show your reasoning.
 \(0.151  0.028\)
 \(0.106  0.0315\)
 \(3.572  2.6014\)
Solution
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(From Unit 3, Lesson 15.)Problem 5
Find these quotients. Show your reasoning.
 \(24.2 \div 1.1\)
 \(13.25 \div 0.4\)
 \(170.28 \div 0.08\)
Solution
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(From Unit 3, Lesson 20.)