Suppose studies indicate that the earth's vegetative mass, or biomass for tropical woodlands, thought to be about 35 kilograms per square meter (kg/m2), may in fact be too high and that tropical biomass values vary regionally—from about 4 to 56 kg/m2. Suppose you measure the tropical biomass in 100 randomly selected square-meter plots. (a) Approximate σ, the standard deviation of the biomass measurements (in kg/m2). σ = kg/m2 (b) What is the probability that your sample average is within two units of the true average tropical biomass? (Use your answer from part (a) in your calculations. Round your answer to four decimal places.)