Hint



Click the box that contains the correct rule for each sum.
Add 0
Add 1
Add 2
23 +
= 23
18 +
= 20
28 +
= 29
38 +
= 39
38 +
= 40
38 +
= 38
83 +
= 85
90 +
= 90
78 +
= 79