That's exactly how I started teaching my 2nd grader. They got some problems like that for their homework and on the scratch paper I said, "well for the blanks we can just stick an x in there" and she didn't even blink and went with it. Because well blanks look just odd. They might look like a "-" if it is written too high.

This is an example of them taking something that's basic and making it more complicated and cumbersome by trying to "simplify" it.