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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: 

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:

Leslie’s wooden sticks

Her brother’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 ten is represented as:

1 one is represented as:

1 one is represented as:


Also 10 ones combine to make 1 ten:   

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. 

answer 27

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

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

answer 43

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.  


4 ones 3 tens 1 one

Further it is equal to:  

7 tens 5 ones

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

Q: Add 18 and 37  


Q: Add 18 and 37 answer


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. 


  • Find the sum of the following questions with the help of the blocks.
exercise 1
  • Add the following:
exercise 2
  • In a classroom, there are 21 boys and 18 girls. How many students are there in the classroom in all?
exercise 3
  • 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.
exercise 4
  • 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?
exercise 5

Concept Map

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


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