`. - Core answer: `90`. Let's carefully format the MathML and HTML. Let's write out the step-by-step mathematical reasoning:

The correct answer is 90.

To solve this problem, we can break it down into logical mathematical steps by defining an unknown variable for the original number.

Step 1: Define the unknown number and set up the equation.

Let the original number be represented by $x$ .

According to the given problem, 20% of this number plus 90 equals the number itself.

Writing 20% as a decimal gives $0.20$ or as a fraction $\frac{20}{100}$ .

So, we can formulate the equation as:

$0.20 x + 90 = x$

Step 2: Solve for 80% of the number directly.

Rearranging the equation to bring all terms containing $x$ to one side:

$90 = x - 0.20 x$

Subtracting $0.20 x$ from $1 x$ leaves:

$90 = 0.80 x$

Notice that $0.80 x$ represents exactly 80% of the original number!

Therefore, 80% of the same number is directly equal to 90.

Let's double-check all tags and rules: - No LaTeX used. - MathML used `

`, ` `, ``, ``, `\frac{`.
- No `display="block"` anywhere.
- Every line containing `}{...}

If 20% of a number is added to 90, then the result is the same number. 80% of the same number is:

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