2.4 Mental Math: Use the Properties (p.42-45)

Lesson 2.4 is practice for applying the properties knowledge we learned in lesson 2.3. Today we learned about one new strategy called compensation

I. Compensation
A. Used for addition and subtraction
1. For addition: change one number to a multiple of 10 and then adjust the other number by
doing the opposite to keep the balance.
Example: 44 +57 = (44 +6) +(57 - 6) = 50 + 51 = 101
Because 50 + 51 is easier to add together.

2. For subtraction: change each number in the same way, so that the last number being
subtracted is a multiple of 10 that ends in 0.
Example: 128 - 56 = (128 +4) - (56 +4) = 132 - 60 = 72
II. Divide Mentally
A. Divide a number by breaking it into smaller,more managable parts.
Example: 396 ÷ 4
396 = 360 + 36 (break it into parts that can be divided by 4)
360 ÷ 4 = 90 and 36 ÷ 4 = 9 (mentally divide the parts by 4)
90 + 9 = 99
So, 396 ÷ 4 = 99.