68% of the time, values will be within ONE std dev of the mean.
Thus, if the mean is 50 and the std. dev is 5, values will be between 45 and 55.
45 is one std dev below the mean, whereas 55 is one std dev above.
40 and 60 represent 2 std dev below the mean and above the mean, respectively. 95% of the time, values will be within the interval (40,60).
Here we're making use of the so called EMPIRICAL RULE.
99.9% of the time, values will lie within 3 std dev of the mean. This comes out to (45, 65). Unsure of how to conclude the 3rd question here.