3. a civil engineer is analyzing the compressive strength of concrete. compressive strength is normally distributed with σ2 = 1000 (psi)2 . a random sample of 12 specimens has a mean compressive strength of 3250 psi. (a) construct a 95% two-sided confidence interval on the mean compressive strength. (b) suppose it is desired to estimate the mean compressive strength with an error that is less than 15 psi at 99% confidence. what is the sample size required?

In a unit cell of fcc structure, indicate the position of one of the tetrahedral interstitial sites by plotting the corresponding tetrahedron. do the same for one of the octahedral interstitial sites (in a separate fcc unit cell). mark the positions of all the tetrahedral and octahedral interstitial sites in the unit cells. how many equivalent tetrahedral sites and how many equivalent octahedral sites are contained in a fcc unit cell, respectively. what is the coordination number for a tetrahedral site and what is the coordination number for an octahedral site?