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# Add Numbers by Partial Sum using Mental Math

Sep 27, 2022

## Key Concepts

• The sum of 2-digit numbers
• Use models to represent 2-digit numbers
• Explain to add 2-digit numbers with the help of models

## Add 2-Digit Numbers Using Models

Two-digit numbers:

Q: Identify the tens digit and ones digit in the following numbers.

Tens digit in 82 is 8 and ones digit in 82 is 2.

Tens digit in 14 is 1 and ones digit in 14 is 4.

Q: Break apart the following numbers into tens and ones.

Q: Use mental math to find the sum:

Q: Leslie has 21 wooden sticks. Her brother has 13 wooden sticks. Find the total number of sticks with them.

Leslie’s wooden sticks:

Her brother’s wooden sticks:

So, the total number of wooden sticks are 3 tens and 4 ones = 34

### Representation of 2-digit number using the blocks

A 2-digit number has a tens place and ones place.

Here the ones place is represented as one block and tens place is represented as combination of 10 blocks as shown below:

1 ten is represented as:

1 one is represented as:

Also 10 ones combine to make 1 ten:

Q: Represent the number 27 using place value block.

Answer: The number 27 can be written as 2 tens and 7 ones.

Q: Represent the number 43 using a place value block.

Answer: The number 43 can be written as 4 tens and 3 ones.

### Sum of 2-digit number using the blocks

Using the blocks let us find the sum of 2-digit numbers by just adding tens blocks and ones blocks separately with the given example below:

Q: Find the sum of 44 + 31.

Further it is equal to:

Now let’s, find the sum of 2-digit number by regrouping.

#### Activity

Sam has 51 pencils. His brother Ron has 36 pencils. How many pencils do they have in all? Use place value blocks to help yourself solve. Show your place-value blocks.

## Exercise

• Find the sum of the following questions with the help of the blocks.
• In a classroom, there are 21 boys and 18 girls. How many students are there in the classroom in all?
• Richard scored 88 runs in the first match and 62 in the second match in the inter-school games. Find the total number of runs scored by him in the two matches.
• In a library there are 58 books in a bookshelf. 18 more books are put in the book shelf later. How many books are there in all?

### What have we learned

• Recall the sum of 2-digit numbers
• Use Models to represent 2-digit numbers
• Add 2-digit numbers by using models

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