78 plus what equals 90?

This is how to calculate "78 plus what equals 90?" In other words, 78 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:

78 + what = 90

To solve this equation, we first add -78 to each side of the equal sign like this:

78 - 78 + what = 90 - 78

Then, we do the calculation to get the answer to "78 plus what equals 90?" as follows:

What = 12

The answer to "78 plus what equals 90?" is 12. You know this answer is correct because 78 plus 12 equals 90.

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78 plus what equals 91?
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