For this case we are going to use the following variables:
y = amount in $ of tip.
x = amount in $ that Nina must pay to eat a meal.
The mathematical expression will be
y = 0.18x
For example if Nina pays $ 100 for a meal, then the tip is:
y = 0.18 * (100)
y = 18 $
Answer:
An expression represents the amount of tip, in dollars, that Nina will give when she has a positive experience eating a meal that cost n dollars is:
y = 0.18x
