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.
Relating addition and subtraction by Khan Academy
Relate addition and subtraction by Khan Academy
Conclude by leading this investigation:
Cookie Monster Problem
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.