81 plus what equals 90?

This is how to calculate "81 plus what equals 90?" In other words, 81 plus a number equals 90. What is that number? We start by converting this word problem into an equation where "what" is what we want to solve:

81 + what = 90

To solve this equation, we first add -81 to each side of the equal sign like this:

81 - 81 + what = 90 - 81

Then, we do the calculation to get the answer to "81 plus what equals 90?" as follows:

What = 9

The answer to "81 plus what equals 90?" is 9. You know this answer is correct because 81 plus 9 equals 90.

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81 plus what equals 91?
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