280 plus some number plus 388 equals 280 plus 244 plus 388.
We have a missing value, but our equal sign tells us that whatever is on the left side of this equation has to be the same as what is on the right side of this equation. Another thing we can check is what operations we’re working with here. We’re only working with addition. The associative property of addition tells us that when we’re working with addition, it doesn’t matter which numbers you add first — that our grouping here doesn’t matter as long as we’re working with the same numbers on both sides.
On the left side, I see 280 and on the right side we have 280. On the left side, we have 388, and on the right side, we have 388. The right side has one remaining value and the left side has the missing value. Our missing value has to be 244. Because of our knowledge of the associative property of addition, we can confidently say that 244 is the missing value from this equation.