The lumber used to frame walls of houses is frequently sold in-multiples of 2 feet. If the length of a board is not exactly a multiple of 2 feet, there is often no charge for the additional length. For example, if a board measures at least 8 feet but less than 10 feet, then the consumer is charged for only 8 feet.

(a) Suppose that the cost of lumber is $0.80 for every 2 feet. Find a formula for a function f that computes the cost of a board x feet long for 6 ≤ x ≤ 5 18.

(b) Graph f.

(c) Determine the costs of boards with lengths of 8.5 feet and 15.2 feet.