#### Need Help?

Get in touch with us

The component learnSearchBar has not been created yet.

# Add Numbers by Partial Sum using Mental Math

## 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

#### Composite Figures – Area and Volume

A composite figure is made up of simple geometric shapes. It is a 2-dimensional figure of basic two-dimensional shapes such as squares, triangles, rectangles, circles, etc. There are various shapes whose areas are different from one another. Everything has an area they occupy, from the laptop to your book. To understand the dynamics of composite […] #### Special Right Triangles: Types, Formulas, with Solved Examples.

Learn all about special right triangles- their types, formulas, and examples explained in detail for a better understanding. What are the shortcut ratios for the side lengths of special right triangles 30 60 90 and 45 45 90? How are these ratios related to the Pythagorean theorem?  Right Angle Triangles A triangle with a ninety-degree […] #### Ways to Simplify Algebraic Expressions

Simplify algebraic expressions in Mathematics is a collection of various numeric expressions that multiple philosophers and historians have brought down. Talking of algebra, this branch of mathematics deals with the oldest concepts of mathematical sciences, geometry, and number theory. It is one of the earliest branches in the history of mathematics. The study of mathematical […]   