Here is a quick overview of what we have been practicing so far in our Addition and Subtraction unit. The best way for you to see how your child is doing with these concepts is to have a conversation about them and do a few practice examples together
1. Doubles and Near-Doubles Addition Facts: We spent quite a bit of time practicing our doubles facts as a class and individually. We focus on doubles because they are often easy to learn and can be a helpful mental math strategy for solving other addition facts that are close to the double. Students practiced solving “near doubles” equations and explaining the strategy as clearly as they can.
Example: (students are asked to show that they can use both of the possible doubles to solve each problem)
I know 6+6=12
6+8 is 2 more.
I know 8+8=16
6+8 is 2 less.
2. Making 10:
Example: To solve 6+8 we may think: Take 2 from the 6, leaving 4. Add 2 to the 8 to make 10, then add the 4 to get 14.
Or, use an adding a 10 fact like the example below.
We know 10+6=16.
8+6 is 2 less.
We have also been practicing our addition facts that make 10.
3. Related Addition and Subtraction Facts (Fact Families):
Example: If we know 7+6=13 we also know 6+7=13, 13-6=7, 13-7=6. Using “fact family” knowledge is often most useful for students to solve subtraction equations by thinking of the related addition fact.
4. Estimating Sums: We use the following three strategies to help us build mental math skills.
1. Add only the digits in the tens place.
So, 59+23 is estimated to be 70.
2. Take one number to the closest 10.
So, 59+23 is estimated to be 73.
3. Take both numbers to the closest 10.
So, 59+23 is estimated to be 80.
We focus on learning the strategies and practice deciding which strategies gives the best estimate based on the numbers presented in the problem.
5. Three Digit Addition:
We continue to work hard at maximizing our learning about numeracy while exploring addition and subtraction of three digit numbers. The students have many strategies for working with three digit number equations. The samples below are examples of some of the strategies we have been working with. Please keep in mind that these are just basic examples and individual students will often take apart and re-construct the numerals in unique ways that match their understanding of numbers. When this happens it is often a very good sign that they are good mathematical thinkers. There is a big difference between blindly following steps to get the correct sum and being able to make efficient adjustments to strategies based on the problem that is presented.
300 + 200 = 500 (add the hundreds)
60 + 50 = 110 (add the tens)
8+7 = 15 (add the ones)
500 + 110 = 610 (add your answers from the 100s, and 10s)
610 + 15 = 625 (add answer from the 1s the sum of the 100s and 10s)
500 (add the hundreds)
110 (add the tens)
_15 (add the ones)
For both addition and subtraction some students need to build/draw base 10 materials to help them visualize what is happening to solve three digit problems.