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. 

  

Answer:   

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

Answer:  

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

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

Answer:  

Q: Use mental math to find the sum: 

Answer:  

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:

  

Answer:  

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.  

Answer:  

Further it is equal to:  

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

Q: Add 18 and 37  

Answer:  

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.

• Add the following:

• 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?

Concept Map

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