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Mental Math within 100
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 How to do mental math with numbers up to 100.
 How we can use place value to add and subtract in our heads.
 Explore how doing mental math can help us build a tower, color a rainbow, and take care of mice!

Discussion Questions

Before VideoWhat does mental math mean?ANSWER

Doing math in your head without pencil and paper.

Using manipulatives, drawing, charts, and writing the numbers by lining them up by place value.

10 + 5 is easier to add because 10 is an easy number to use. To add, we just change the ones place.

A number that is easier to add or subtract with, like 10, 20, or 30. Skip counting means adding the same number over and over, which is also easier.

We can ask “5 plus what number is equal to 9?”


After VideoHow can we use mental math to add 6 + 9?ANSWER

We can subtract one from 6 and add one to 9. Then we have 5 + 10. We can also first add 4 to 6, so that we have 10, and then we can add the remaining 5, so we have 10 + 5 = 15. There is more than one way.

Yes. We can count on from 17: 18, 19, 20. We counted 3 numbers, so 20 – 17 = 3.

We can first subtract 1 from 11 to make 10. Then subtract 3 more so we are subtracting 10 – 3, which is easier. We can also count on from 4 to 11.

The tens place changes but the ones place stays the same.

The tens place changes but the ones place stays the same.



Vocabulary

Add
DEFINE
To put amounts together.

Plus
DEFINE
The word used to describe adding two amounts.

Subtract
DEFINE
To find the difference between two amounts.

Minus
DEFINE
The word used to describe subtracting two amounts.

Mental Math
DEFINE
Solving problems without paper and pencil.

Friendly Numbers
DEFINE
Numbers that are familiar and work as easy starting points, like 10 and 20.

Skip Counting
DEFINE
Counting by adding the same number over and over again.

Add
DEFINE

Reading Material
Download as PDF Download PDF View as Separate PageWHAT IS MENTAL MATH WITHIN 100?Mental math is math that you do without using a pencil and paper. Friendly numbers and skip counting can help you add and subtract using mental math.
To better understand mental math within 100…
WHAT IS MENTAL MATH WITHIN 100?. Mental math is math that you do without using a pencil and paper. Friendly numbers and skip counting can help you add and subtract using mental math. To better understand mental math within 100…LET’S BREAK IT DOWN!
What are friendly numbers?
Friendly numbers are numbers that are easy to add. It is easy to add 10 and another number. 10 + 2 = 12. It is easy to add together numbers that make a ten. 4 + 6 = 10. Doubles are also easier to add. 6 + 6 = 12.
What are friendly numbers? Friendly numbers are numbers that are easy to add. It is easy to add 10 and another number. 10 + 2 = 12. It is easy to add together numbers that make a ten. 4 + 6 = 10. Doubles are also easier to add. 6 + 6 = 12.Find how many crayons in all.
You have 11 crayons. A friend has 7 crayons. How many crayons in all? 11 + 7 = ? To make friendlier numbers, take 1 away from 11 to get 10. Then move 1 to the 7 to get 8. 10 + 8 = 18. There are 18 markers in all.
Find how many crayons in all. You have 11 crayons. A friend has 7 crayons. How many crayons in all? 11 + 7 = ? To make friendlier numbers, take 1 away from 11 to get 10. Then move 1 to the 7 to get 8. 10 + 8 = 18. There are 18 markers in all.Find how many flowers in all.
You have 8 red flowers and 9 blue flowers. How many flowers in all? Use a double fact: 8 + 8 = 16. 8 + 9 is one more, so 8 + 9 = 17.
Find how many flowers in all. You have 8 red flowers and 9 blue flowers. How many flowers in all? Use a double fact: 8 + 8 = 16. 8 + 9 is one more, so 8 + 9 = 17.Find how many mice are left.
There are 24 mice in a basket. 11 mice leave. How many are left? Find 24 – 11. It is easy to subtract 10. 11 is 10 and 1 more. 24 – 10 = 14. 14 – 1 = 13. There are 13 mice left.
Find how many mice are left. There are 24 mice in a basket. 11 mice leave. How many are left? Find 24 – 11. It is easy to subtract 10. 11 is 10 and 1 more. 24 – 10 = 14. 14 – 1 = 13. There are 13 mice left.Towers
There is a tower of 20 blocks. You add stacks of 10 blocks on top. To add groups of 10 you can skip count by 10. 20 + 10 = 30. To add 34 + 50, skip count by 10. Start at 34, then jump to 44, 54, 64, 74, 84. So, 34 + 50 = 84.
Towers There is a tower of 20 blocks. You add stacks of 10 blocks on top. To add groups of 10 you can skip count by 10. 20 + 10 = 30. To add 34 + 50, skip count by 10. Start at 34, then jump to 44, 54, 64, 74, 84. So, 34 + 50 = 84. 
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Add 8 + 7 in your head. What are the steps?
Subtract 17 – 9 in your head. What are the steps?
Add 17 + 40 in your head.
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