If I had to fill in the blank, I'd probably do it like this.

49 + 1 makes 50, and 28 more make 78. Since 1 + 28 is 29, the blank gets filled with 29 and the complete sentence is

49 + 29 = 78

A third grader might not jump to 28, but might add by 10s to get 60, 70, and then 8 more to get 78. If one were counting out change, one might count

49 + 1 makes 50, 8 more make 58, and 20 makes 78. The change is 1+8+20 = 29. A certain amount of planning and short-term memory are involved.

_____

Fundamentally, this is algebra. Somehow people forget how to do it from 3rd grade to 7th grade, or they get shaken by the use of a variable instead of a blank. 49+x = 78, x = 78-49 = 29.