The Zinger is a modern sports car with an epoxy body. The manufacturer constructs Zinger bodies under pressure in a mold, and must wait a certain length of time to be sure that enough resin bonding and hardening have occurred before removing the body from the mold. Chemical engineers have found that the hardening time in normally distributed with mean 28.7 hours and standard deviation 6.4 hours. What is the minimal length of time the body should stay in the mold if the manufacturer wants to be 85% sure that the bonds have hardened?