A soft drink manufacturer claims that cans of soda contain , on average, 12 ounces. The machine that fills the cans actually delivers a variable amount of soda. The amount varies randomly according to a normal distribution with standard deviation 0.16 ounce. If you purchase a six-pack of sodas, and measure the amount in each can and find the average, the probability that the average of the six cans will be between 11.9 and 12.1 oz equals which standard normal probability?