Coffee machines impact customer satisfaction through drink quality.

The function used to calculate drinkQuality (f(x) = 2*x-12) is not
arbitrary. The maximum value z can have such that if avg(x,y,z) < 0,
then min(x,y,z) ≠ z is z=-2. This implies that if drinkQuality > -2,
customers cannot have a negative quality thought bubble. The previously
mentioned linear function is the closest approximation such that both x
and f(x) are whole numbers. This means that when the customer has
bought the most expensive machine (x=4), the drink quality will be at
least -4 (since f(x)= -4), which means that the customer COULD have a
negative quality thought bubble. However, the probability of this
happening is very low.

tl;dr the function used to calculate quality works.