Thornton Rentals can purchase a van that costs $144,000; it has an expected useful life of four years and no salvage value. Thorntor uses straight-line depreciation. Expected revenue is $59,544 per year. Assume that depreciation is the only expense associated with this investment. Required a. Determine the payback period. (Round your answer to 1 decimal place.) b. Determine the unadjusted rate of return based on the average cost of the investment. (Round your percentage answer to 1 decimal place (i.e., 0.234 should be entered as 23.4).)