The monthly cost of driving a car depends on the number of miles driven. Lynn found that in May it cost her $505 to drive 460 mi, and in June it cost her $540 to drive 600 mi.

a) Calculate the cost per mile for May and June.
b) Analyze the trend in the cost of driving.
c) Predict the cost for driving 800 mi in July.
d) Discuss potential factors influencing the cost variation.