A researcher is interested in whether the color of an animal's surroundings affacts learning rate. She tests 16 rats in a box with colorful wallpaper. Assume it is known that the average rat can learn to run this particular maze (in a box without any special coloring) in 25 trials, with a variance of 64; the distribution is normal. The mean numbers of trials to learn maze for the group with the colrful wallpaper is 11. What is the standard deviation of the distribution of means?