To find the selling price of the office chair after marking it up by 28%, we need to use the original cost and include the markup.
Let the cost of the chair be \( C = 63 \) dollars. The markup as a percentage of the cost is 28%. The selling price \( S \) can be calculated using the following equation:
\[ S = C + (0.28 \times C) \]
This can also be expressed as:
\[ S = C \times (1 + 0.28) = C \times 1.28 \]
Thus, the correct equation to find the selling price of the office chair is:
\[ S = 63 \times 1.28 \]
From the options given, the relevant multiplier for the markup which you would use in the calculation is:
X = 1.28