The average life of a bread-making machine is 7 years, with a standard deviation of 1 year. assuming
that the lives of these machines follow approximately a normal distribution, find
(a) the probability that the mean life of a random sample of 9 such machines falls between 6.4 and 7.2 years;
(b) the value of x to the right of which 15% of the means computed from random samples of size 9
would fall.