Missing addend with x or a box?

Recently I've been writing worksheets for fourth grade... Later this year, I will make this collection available for whoever wishes to buy it. But for now, it's a work in progress.

One topic I was pondering a lot was whether to include problems such as

15 + x = 30 into the worksheets.
Or, 234 + x = 700
or x + 1,923 = 5,000.

I hope you get the idea; in elementary books you often see these kind of missing addend problems with a little box:

15 + = 30 or
234 + = 700 or
+ 1,923 = 5,000.

Well, I decided for the x over the box! I don't think solving missing addend problems with subtraction is too difficult to learn on fourth grade; after all, students have been working with addition and subtraction connection from 1st grade on, right?

In my own old schoolbooks I actually see x from 3rd grade on.

I made several problems with charts such as

`Write a missing addend sentenceand a subtraction sentence to solve it.         1,500|------------|-----|     x         346`

Hopefully those help the students with algebraic thinking.

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