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Mathematics
Quarter 1 – Module 11(a):
Adding Mentally 2-digit
and 1-digit Numbers
 What I Know

Read and analyze the questions. Choose the letter of the

correct answer. Write the chosen letter on a separate sheet of
paper.

1.What is the sum of 62 + 5?

a. 67 b. 37 c. 55 d. 70

2.What number is added to 56 to get 63?

a. 7 b. 4 c. 6 d. 5

3.If you add 29 and 5, you get a sum of ______.

a. 34 b. 43 c. 41 d. 36

4.Which addition can be performed without regrouping?

a. 13 + 8 b. 56 + 9 c. 17 + 7 d. 11 + 8

5. Which addition can be performed with regrouping?

a. 13 + 5 b. 56 + 3 c. 13 + 7 d. 11 + 2

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 Adds mentally 2-digit and 1-
digit number with or without
Lesson
regrouping using
appropriate strategies

Most children like you enjoy playing mind games like number
puzzle that applies mental addition. In this module, you will learn
how to add mentally 2-digit and 1-digit numbers with or without
regrouping using appropriate strategies.

What’s In

When we say mental addition, this means that we will add

numbers without writing the solution in the paper. Instead, we do
the process of addition in our mind only.
To make mental addition easier, it is important to master the
process of estimating sums which is taken in your previous lesson.
Let us take a short review.
Example: Estimate the sum of 68 + 72 + 23
Step 1 Round off the addends to the tens place.
68 70, 72 70, 23 20
Step 2 Add the rounded off numbers.
70
+ 70
20
160 The estimated sum is 160.

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Activity 1
Estimate the sum of the following sets of addends.
1.) 58 + 36 3.) 29 + 49 5.) 77 + 11
2.) 16 + 66 4.) 18 + 68

What’s New

The first step in estimating the sum you did in Activity 1 is to

round off the 2-digit numbers to its nearest tens. Notice that when
the addends are in numbers by ten, it makes addition easier.
Let us practice adding mentally involving numbers in tens.

Activity 2
Add mentally.

1. 10 + 9 = _____ 6. 20 + 2 = _____
2. 50 + 5 = _____ 7. 80 + 4 = _____
3. 3 + 90 = _____ 8. 1 + 30 = _____
4. 8 + 30 = _____ 9. 6 + 40 = _____
5. 70 + 7 = _____ 10. 50 + 3 = _____

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 What is It

To add mentally 2-digit and 1-digit numbers, follow these

simple steps:
Step 1. Expand the 2-digit addend into tens and ones
mentally.
Step 2. Add mentally the tens and ones separately.
Step 3. If regrouping is necessary, expand the new addend
into tens and ones and repeat step 2.

Example 1: Add mentally 63 + 5.

Solution:
Step 1. Expand the 2-digit addend. 60 + 3 + 5
Step 2. Add the tens and ones. 60 + 8
Step 3. Add. 68
Answer: 63 + 5 = 68

Example 2: Add mentally 78 + 9.

Solution:
Step 1. Expand the 2-digit addend. 70 + 8 + 9
Step 2. Add the tens and ones. 70 + 17 (with grouping)
Step 3. Expand the new addend. 70 + 10 + 7
Step 4. Add the tens and ones. 80 + 7
Answer: 78 + 9 = 87

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 What’s More

Find the sum mentally.

1.) 13 2.) 35 3.) 61 4.) 76 5.) 43
+ 5 + 4 + 7 + 1 + 2

6.) 44 7.) 38 8.) 17 9.) 26 10.) 25

+ 7 + 9 + 7 + 8 + 5

What I Have Learned

To add mentally 2-digit and 1-digit numbers, follow these

simple steps:
Step 1. Expand the 2-digit addend into tens and ones
mentally.
Step 2. Add mentally the tens and ones separately.
Step 3. If regrouping is necessary, expand the new addend
into tens and ones and repeat step 2.

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 What I Can Do

Add mentally.

1.) 54 + 6 = 6.) 72 + 4 =

2.) 50 + 3 = 7.) 67 + 7 =

3.) 21 + 9 = 8.) 34 + 2 =

4.) 23 + 5 = 9.) 23 + 8 =

5.) 13 + 8 = 10.) 88 + 8 =

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 Assessment

Add mentally. Choose the letter of the correct answer. Write

your answer on a separate sheet of paper.

1.) What is the sum of 23 and 8?

A. 13 B. 31 C. 238 D. 28

2.) What number is added to 56 to get 63?

A. 7 B. 4 C. 6 D. 5

3.) If you add 29 and 6 you get a sum of what number?

A. 53 B. 30 C. 35 D. 96

4.) Oscar gave Marlon 12 big marbles and 9 small marbles. How
many marbles did Marlon received?
A. 12 B. 29 C. 21 D. 129

5.) Which of the following has a sum of 65?

A. 53 + 8 B. 56 + 9 C. 57 + 6 D. 55 + 7

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 Additional Activities

A. Add mentally.

1.) 45 + 5 = ____ 3.) 31 + 9 = _____ 5.) 12 + 7 =_____

2.) 24 + 7 = ____ 4.) 22 + 5 = _____ 6.) 35 + 6 = ____

B. Read and answer the problem mentally.

1.) What is the sum of 23 and 2 added to the sum of 24 and 7?

2.) The sum of 61 and 2 is added to 7. What is the answer?

3.) Rose reads 10 pages of a book. She reads 9 pages of the

same book the next day. How many pages of books did
she read in all?

4.) There are 15 boys and 9 girls enrolled in Grade 3. How

many pupils are there in all?

5.) RB and Jansen are brothers. Both received a box of

marbles from their mother as a reward for helping her clean
the yard. RB counted 10 marbles in his box. Jansen
counted 9 marbles. How many marbles do they have in all?

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 10
What I Know What's In What’s More
1. A
2. A 1. 18
1. 60 + 40 = 100 2. 39
3. A
2. 20 + 70 = 90 3. 68
4. D
3. 30 + 50 = 80 4. 77
5. C
4. 20 + 70 = 90 5. 45
5. 80 + 10 = 90 6. 51
7. 47
8. 24
What's New 9. 34
10. 30
1. 19
2. 55
3. 93
4. 38
5. 77
6. 22
7. 84
8. 31
9. 46
10. 53
Additional Activity
What I Can Do Assessment
A. B.
1. 60 1. B
2. 53 2. A
1. 50 1. 56
3. 30 3. C
2. 31 2. 70
4. 28 4. C
3. 40 3. 19
5. 21 5. B
4. 27 4 .24
6. 76
5. 19 5. 19
7. 74
6. 41
8. 36
9. 31
10. 96
Answer Key