1.OA.B.4
Understand subtraction as an unknown-addend problem. For example, subtract 10 - 8 by finding the number that makes 10 when added to 8.
Solve subtraction problems by thinking about what needs to be added to the subtrahend to reach the minuend. For example, 10 - 8 = __ can be solved by thinking about what needs to be added to 8 to reach 10. You could start your lesson by having students watch this video.