Lesson: Reasonableness of Answer
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Lesson Objective
SWBAT judge the reasonableness of an answer by using estimation.
Lesson Plan
Materials Needed: DN worksheet (cut in half), dry erase markers, white board, blank paper, pencils, and IND Worksheet.
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Do Now (3 5 min): Teacher gives each student a DN worksheet and asks them to complete the work as quickly and quietly as possible. The students should only complete half of the worksheet.
Opening (2 3 min): Teacher quickly reviews answers to the Do Now and then says, “We have been learning about using estimation in order to solve addition and subtraction problems. Today, we are going to use estimation to pick the correct answer from options presented. By the end of this lesson, you will be able to judge the reasonableness of an answer by using estimation.”
Direct Instruction (10 min): Teacher begins “Ok! So before we begin let’s review all the strategies we can use to estimate. [front end rounding, rounding, regrouping] YES! those are the three ways we have reviewed to estimate. Today it is going to be up to you to decide with strategy you use, because the problems that you will be given don’t call for you to estimate. BUT using estimation can help you find the answer more quickly and more accurately. Watch as I do this. I was given the following problem on the DCCAS. Now yes I can easily add this problem. However, if I round first and add, I will be able to eliminate answers and increase my probability of selecting the correct answer. ”
1,789 + 2,890 =
A: 3,720
B: 4,567
C: 4,679
D: 5,500
The teacher continues, “Ok my first step is to estimate, but I have to decide which place I will round to estimate. After looking the answer choices I can see that that estimating the thousands place wouldn’t give me enough information because there are 2 answer in the 4000, so what if I estimated the thousands I get 4000, and that doesn’t help me narrow down the answer choices to 1. So I am going to estimate to the hundreds place. Estimate: 1,800 + 2,900= 4,700, Ok, so now I can cross off answer choice A and B. For this particular problem I can either estimate to the tens place, or just add up the numbers to arrive the correct answer, which is C”
The teacher then completes one more example with the students before moving to guided practice.
57 x 32 = ?
A: 1,514
B: 2,100
C: 1,765
D: 1,824
The teacher should ask the student what digit the problem should be rounded to based on the answer choices. Estimate 60 x 30 = 1,800. The teacher should ask the students which answers they could cross off based on their estimation. The teacher should pose the question: “Which is the most reasonable answer? If we only had 15 seconds to decide, which would be the most reasonable answer?” The answer is D.
Guided Practice (810 min): Teacher says, “Now, I will give you four problems to do together in groups of 2 or 3 on your own paper. I would like to see your estimation, then the answer choices you crossed off, and finally the correct answer circled based on your estimation.” Teacher writes the following on the board and gives each student time to solve the problems together in small groups.
1. 7,809 – 2,379 =
A: 5,430
B: 5,370
C: 5,500
D: 5,410
Estimation: 7,800 2,400 = 5,400
Correct Answer: A
2. 992 / 32 =
A: 23
B: 31
C: 37
D: 46
Estimation: 1,000 / 30 = 33.3
Correct Answer: B
3. 462 x 98 =
A: 44,316
B: 43,818
C: 47,668
D: 45,766
Estimation: 460 x 100 = 46,000
Correct Answer: D
4. 84,132 + 29,207 =
A: 110,339
B: 114,339
C: 113,339
D: 115,339
Estimation: 84,000 + 29,000 = 113,000
Correct Answer: C
Independent Practice (8 min): The teacher reviews the answers to the Guided Practice with the class and then hands out the IND Worksheet. Teacher should tell the students to work the problems independently.
Closing (23 min): Teacher calls the attention of the students back toward the front of the class to quickly review the answers to the Independent Practice/ ask what we learned about.
Lesson Resources
IND WS Reasonable Classwork 
1

DN lesson 10 Starter / Do Now 