An equation that represents a discount of 18% on a retail price of $55 can be written as follows:
p = 0.82 * 55
In this equation, p represents the new price after the discount has been applied, and 0.82 represents the percentage of the original price that remains after the discount has been applied (100% - 18% = 82%).
Solving this equation, we find that the new price, p, is equal to $45.90.
Note: This equation represents a discount, which means that the final price, p, will be less than the original price. If you want to represent a markdown (i.e., a price increase), you would use a value greater than 1 for the percentage factor (e.g., 1.18 for a markdown of 18%).