5 + __ = 6 --> Six is one more than five.
So, the missing number is one.
2 + __ = 9 --> Nine is seven more than two.
So, the missing number is seven.
9 + __ = 12 --> Twelve is three more than nine.
So, the missing number is three.
NOTE:
Make up several more problems like these examples and help the student to
solve them.