21 plus what equals 20?

This is how to calculate "21 plus what equals 20?" In other words, 21 plus a number equals 20. What is that number? We start by converting this word problem into an equation where "what" is what we want to solve:

21 + what = 20

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

21 - 21 + what = 20 - 21

Then, we do the calculation to get the answer to "21 plus what equals 20?" as follows:

What = -1

The answer to "21 plus what equals 20?" is -1. You know this answer is correct because 21 plus -1 equals 20.

Plus What Equals Calculator
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21 plus what equals 21?
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