To gain information about competitors’ products, companies sometimes employ “reverse engineering,” which consists of buying the competitor’s product, then taking it apart and examining the parts in great detail. Engaging in this practice, a bicycle manufacturer intends to buy two or more of a leading competitor’s mountain bikes and measure the tensile strength of the crossbar portion of the frame. Past experience has shown these strengths to be approximately normally distributed with a standard deviation of 20 pounds per square inch (psi). If the bike purchaser wants to have 90% confidence that the sampling error will be no more than 5 psi, how many of the competitor’s mountain bikes should be purchased for destructive testing?