6. (6 points) The mean life span of Americans is 79 years (2012, CDC) with a standard devi- ation 15 years (ID. Edwards, NBER, 2008). (a) (2 pta) Without assume any distribution, use the Chebyshev's rule estimate how many percentage of Americans live in between 49 years and 109 years approximately? (b) (2 pta) If we assume the life span is normal, use the empirical rule: estimate what is the middle 95% range of Americans' life span? (c) (2 pts) If we assume the life span is normal, how many percentage of Americans live more than 109 years approximately