# Estimate Sums by Rounding

Videos to help Grade 3 students learn how to estimate sums by rounding and apply to solve measurement word problems.

Common Core Standards: 3.NBT.2, 3.NBT.1, 3.MD.1, 3.MD.2

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Lesson Plans and Worksheets for Grade 3

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New York State Common Core Math Module 2, Grade 3, Lesson 17

Application Problem

The doctor prescribed 175 milliliters of medicine on Monday and 256 milliliters of medicine on Tuesday.

a. Estimate how much medicine he prescribed in both days.

b. Precisely how much medicine did he use in both days?

Concept Development

Estimate the sum of 362 + 159 by rounding.

What is 362 rounded to the nearest hundred? 400.
What is 159 rounded to the nearest hundred? 200.
What is 400 + 200? 600
We estimated the sum by rounding to the nearest hundred and got 600.

Let’s now round to the nearest 10 and add.
360 + 160 = 520

We’ve learned to round to the nearest ten and hundred before.
Let’s think if there is another way we could round these numbers that would make them easy to add.
They are both really close to a fifty and those are easy for me to add.
What is 363 rounded to the nearest fifty? 350.
What is 159 rounded to the nearest fifty? 150
350 + 150 is 500.

The actual answer 362 + 159 = 521
Rounding to the nearest 10 is the most accurate.
Rounding to the nearest 50 makes it easier to calculate.

Homework

2. Dena reads for 361 minutes during Week 1 of her school's two-week long Read-A-Thon. She reads for 212 minutes during Week 2 of the Read-A-Thon.
a. Estimate the total amount of time Dena reads during the Read-A-Thon by rounding.
b. Estimate the total amount of time Dena reads during the Read-A-Thon by rounding in a different way.
c. Calculate the actual number of minutes that Dena reads during the Read-A-Thon. Which method of rounding was more precise? Why?

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