Trip plan
In this trip, it is relatively understandable that more mile this trip takes (n), more amount of money needed (s). But we will assume that we the distance is 10 miles. Also the more the machine gets better miles per gallon (k), the less amount of money needed (s), and therefore s indirectly correlated to k. s = 1/k. For example if k = 30mpg and if 1 gallon costs $3 then
s = $3*(10m/30mpg) = 1 dollar
if k = 5mpg and if 1 gallon costs $3, and distance is 10miles
s = $3*(10m/5mpg) = 6 dollar