Final answer:
The professor added 15 milligrams of gold to the hand cream, which appears significant numerically but translates to a very thin layer of about 0.00019 mm.
Step-by-step explanation:
To determine how many milligrams of gold were added to the hand cream in the professor's experiment, let us refer to the provided information that 15 milligrams (15 mg) of gold were used. This amount, while numerically significant when stated in milligrams, translates to a very small thickness of about 0.00019 mm. This comparison helps us understand the physical size of the gold quantity in a real-world context.
The mass of gold makes it sound more substantial due to the larger numerical representation, which can be confusing or misleading. In contrast, millimeters are a more commonly understood measurement for most people, allowing them to grasp the thinness of the gold layer more intuitively. It is important for consumers to analyze information critically, especially when it's used in advertising, to understand the true significance of what is being claimed or sold.