Strategies to Add and Subtract 3-digit Numbers

 Introduction:

Strategies to Add and Subtract:

In this session, we will discuss the addition and subtraction of 3-digit numbers by using different strategies.

1. Bar diagram
2. Open number line
3. Partial difference
4. Partial sums
5. Estimation
6. Column addition

Problems Based on Strategies:

Example:

There are 134 fewer cell phone towers in Jurloe Country than in Fraser Country. How many cell phone towers are there in Jurloe Country? Choose a strategy, and then solve it.

Solution:

Step1:

i) Estimation:

404 is about 400.

134 is about 100.

The difference is about 400 – 100 = 300

ii) Open number line:

 Find 404 =? + 134

200 + 66 + 4 = 270

There are 270 cell phone towers in Jurloe Country.

Step 3:

iii) Partial Difference:   

Find 404 – 134 =?

There are 270 cell phone towers in Jurloe Country.

iv) Partial Sums:

134 + 270 =?

v) Column Addition:

Find 404 – 134 =?

vi) Bar Graph:

Find 134 + 270 =?

Problems Based on Partial Sums and Differences:

Example 1:

Sally and Quadra have a spider-catching contest. Sally catches 476 spiders. Quadra catches 349 spiders. How many spiders have they caught altogether?

Solution:

You can estimate and then use place value to add the numbers.

476 is about 500.          500

349 is about 300.       +300

                                    800

Add each place value.

Place value of 349= 300+40+9.

Start with hundreds: 

Partial Sums:

Sally and Quadra caught 825 spiders together.

Example 2:

Dina was adding books to the library shelves. She put 117 nonfiction books on the shelves. Then there were 204 nonfiction books. How many nonfiction books were on the shelves before? Find 204 – 117.

Solution:

Estimate first.

204 – 117 =?

200 – 100 = 100.      

Use place value to subtract.

117 = 100 + 10 + 7

586 = 500 + 80 + 6

Partial Difference:

There were 87 books on the shelves before Dina added more books.

Exercise:

(1 – 2) Estimate:

1. 632 – 218
2. 753 + 317

(3 – 4) Use the partial difference to subtract:              

3. 794 – 355

4. 455 – 182

(5 – 6) Partial sums to add:

5. 249 + 613

6. 642 + 123

(7 – 8) Use column addition:

7. 700 + 64

8. 807 + 65

9. Jada spends $74 on a hat, shoes, and shorts. If the hat cost $22 and the shoes cost $33, how much were the shorts?

10. Don’s book has 315 pages. He read 50 pages last week. He read another 71 pages this week. How many more pages does Don have left to read?

Concept Summary:

What We Have Learned:

• Partial differences using place values
• Partial sums using place values
• Estimation
• Cross addition
• Open number line