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
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