6. A study was undertaken to determine if fire will affect the amount of calcium available to plants and animals. The amount of calcium taken up and stored is closely correlated to the amount present in the soil. A large tract of land in the Fishburn Forest was selected for a prescribed burn. Soil samples were taken from 11 plots of equal area just prior to the burn and analyzed for calcium. Postburn calcium levels were analyzed from the same plots. The calcium levels in kilograms per plot are presented in the following table. Find a 99% confidence interval for the mean difference in calcium levels in the soil prior to and after the prescribed burn. Assume the distribution of differences in calcium levels to be approximately normal. Can we conclude that fire changes the mean calcium level in the soil? Explain. Show your calculations and formulae. Plot 1 2 3 4 5 6 7 8 9 10 11 Preburn 50 50 82 54 82 73 54 23 45 36 54 Postburn 19 18 45 28 50 33 12 10 15 12 20