This method can be done mentally. Have children orally explain how they found the sum. Point out that addition is binary, meaning that only two numbers can be added at any one time. Arrange the written responses in an organized fashion on the board. This is a special pattern called the Associative Property or Grouping Property.
Associates are people who work together.
In the Associative Property, numbers are combined by using parentheses. Look at this poster. Display the poster of the Associative Property. Guide children to realize that using both properties allows them to choose which addends to add first. This allows sums to be found mentally.