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Number and Operations in Base Ten 3.NBT.2

Lesson 4 Add Mentally

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There are 58 passengers on a subway train. At the next stop, 33 more passengers board the train. How many passengers are there altogether?

You can add mentally. Make one addend a ten such as 10, 20, 30, and so on.

58 + 2 + 33 - 2

Give, or add, 2 to the first addend.

Take, or subtract, 2 from the second addend.

So, 60 + 31 = 91. There are 91 passengers.

60 + 31

91

60 and 31 are easier numbers to add.

Practice

Break apart the addends to add mentally.

1. 4 1 = 40 + 1 + 26 = 2 0 + 6

6 7 = 60 + 7

2. 328 = 3 0 0 + 20 + 8 + 254 = 200 + 50 + 4 5 8 2 = 5 0 0 + 7 0 + 12

Make a ten or a hundred to add mentally.

3. 76 + 7

- 3

+ 3

73 + 10 = 83

4. 598 + 256

+ 2

- 2

600 + 254 = 854

Lesson 4My Homework83

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Make a ten or a hundred to add mentally.

5. 339 + 123

+ 1

- 1

340 + 122 = 462

Sample computations: 5, 6

6. 399 + 428

+ 1

- 1

400 + 427 = 827

Brain Builders

Break apart the addends to add mentally. Explain to a friend the strategy you used.

7. 767 = 7 0 0 + 6 0 + 7 8. 21 4 = 2 0 0 + 1 0 + 4

+ 29 =

20 + 9

+ 127 = 1 0 0 + 2 0 + 7

796 = 700 + 80 + 16

341 = 300 + 30 + 11

9.

2 Use Number Sense Lambert's Livery rents canoes

every weekend. How many total canoes were rented in June and

July? Explain how you used mental math to find the sum.

Lambert's Livery

Canoe Rentals

Month Rentals

June

154

July

198

August

176

Sample answer: I added 2 to 198 to get 200, and subtracted 2 from 154 to get 152; 200 + 152 = 352

10. Test Practice During Science Week, there were 77 visitors on Monday and 28 visitors on Tuesday. During Math Week, there were 10 more visitors than during Science Week. How many visited during Math Week?

A 49 visitors B 105 visitors

C 1 1 5 visitors D 21 5 visitors

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