A certain type of thread is manufactured with a mean tensile strength of 78.3kg and standard deviation of 5.6kg. Assuming that the strength of this type of thread is distributed approximately normal, find a) the probability that the mean strength of a random sample of 10 such thread falls between 77kg and 79kg. b) the value of x to the right of which 15% of the means computed from random samples of size 10 would fall.