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
