Mental Math for Addition and Subtraction

1
Mental Math for Addition and Subtraction

• Lesson 2.1

2
Addition Properties

• Commutative Property

• Associative Property

• Numbers can be added in any order.

• Addends can be grouped in different ways,
  resulting in the same sum.

64 78 78 64
(14 54) 6 14 (54 6)
3
Example - Find the Sum
First, lets rearrange the addends.
5 9 21 25
5 25 9 21
Commutative Property
(5 25) (9 21)
Associative Property
30 30
Use mental math
60
So,the sum is 60
4
Compensation

• You can change one number to a multiple of 10
  (make it end in 0) and then adjust the other
  addend to keep the balance.

18 24 (18 2) (24 - 2)
Add 2 to make 20
Subtract 2 to balance
20 22
42
5
Using Compensation to Subtract

• Make the second number a multiple of 10
  (multiples of 10 are numbers that end in 0).
• Subtract the same amount from both numbers.

57 - 33 (57 - 3) - (33 - 3)
Make 33 a multiple of 10 by subtracting 3.
Take away the same amount from the first number.
54 - 30
24
6
Homework Time!