Steve Harrington is caught in an underground cavern that is filling with toxic gas. He can detect its strong metallic odour. At first, the concentration of the gas is 100 ppm. Unsure whether the concentration is increasing or not, he waits a minute and takes another sniff. If his Weber fraction for the scent of the gas is .2, what is minimum level that the concentration will have to increase to in order for him to detect the difference 50% of the time?