For many travelers, the cost of getting to an airport needs to be factored into the travel expense. The table at the right lists the cost of six different ways to get from the center of a major city to the nearest airport. A researcher polled travelers to determine the number of people who paid each price on a given day. Variables a, b, c, d, e, and f represent the frequencies of the costs.

a. Write an algebraic expression for the percentile rank of $25.
b. Write an algebraic expression for the percentile rank of $38.
c. Write an algebraic expression for the percentile rank of $42.
d. Write an algebraic expression for the percentile rank of $60.
e. Write an algebraic expression for the percentile rank of $65.
f. Write an algebraic expression for the percentile rank of $70.