Researchers in microelectronics have found that a slight scratch on the surface of 5 pts. gallium arsenide causes a zinc dopant to diffuse into the arsenide. When these devices are baked at 850°C the dopant spreads. Its diffusion coefficient at this temperature is 10−11cm^2/s. If it spreads enough to increase the zinc concentrationto 10% of the maximum concentrationat 4×10^−4 cm away from the scratch the device is ruined. How long can we safely bake the device?