To calculate the mean and standard deviation of the fat content we need to use the multiplication rule (Z=X/Y) given by Equation 4. Initially, we assign values to the various parameters in the appropriate propagation of error equation: X = 3.1; D X = 0.3. Y = 10.5; D Y = 0.7 % Fat Content = Z = 100 ´ X/ Y = 100 ´ 3.1/10.5 = 29.5%