21 plus what equals 8?

This is how to calculate "21 plus what equals 8?" In other words, 21 plus a number equals 8. What is that number? We start by converting this word problem into an equation where "what" is what we want to solve:

21 + what = 8

To solve this equation, we first add -21 to each side of the equal sign like this:

21 - 21 + what = 8 - 21

Then, we do the calculation to get the answer to "21 plus what equals 8?" as follows:

What = -13

The answer to "21 plus what equals 8?" is -13. You know this answer is correct because 21 plus -13 equals 8.

Plus What Equals Calculator
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