Khan Academy
Khan Academy: Understand Subtraction as Adding the Opposite
Practice rewriting subtraction problems as equivalent addition problems. For example, 2-9=2+(-9). Students receive immediate feedback and have the opportunity to try questions repeatedly, watch a video or receive hints.
Better Lesson
Better Lesson: More Than Two Numbers
The big idea of this lesson is that three (or more) numbers can be grouped together and added. The order of this grouping doesn't matter.
Mr. Martini's Classroom
Mr. Martini's Classroom: Mr. Martini's Classroom: Adding Objects
Practice addition facts 1 to 9 with numbers and pictures in this interactive game.
Better Lesson
Better Lesson: Any Order
The numbers in an addition number sentence (addends) can be added in any order and the sum remains the same.