Solve each problem.

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

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

(b) Determine the costs of boards with lengths of 8.5 ft and 15.2 ft.