2The Basic Steps: Line up the numbers by the decimal point. Fill in missing places with zeroes.Add or subtract.
3Addition PropertiesCommutative Property of Addition: Numbers can be added in any order.Example: 5+3=3+5Associative Property of Addition: Numbers can be grouped in any way.Example: (7+3) + 5 = 7 + (3+5)Identity Property of Addition: The sum of any number and zero is that number.Example: = 18
4Mental Math Addition properties can be used to quickly find sums. Example: What is ?We can use an addition property to quickly add this.First, change the order: ________________ property.Then, group the first two addends together: ( ) ________________ property= 112.4commutativeassociative
5Why Estimate?Before adding your numbers, use one of the estimating strategies you learned to see what the answer should be close to.When you add, you can see if your answer is reasonable.
6Example:This can estimate to , which is 43. (Compatible numbers)28.9+13.3142.21The answer is close to the estimate.
7Subtracting The addition properties do not work for subtraction. Estimate and subtract to find what the answer is close to.Subtract to find the real answer.
8Subtracting Across Zeroes If you have several zeroes in a row, and you need to borrow, go to the first digit that is not zero, and borrow.All middle zeroes become 9’s.The final zero becomes 10.