A construction company is designing the world's highest commercial building at the Michigan Avenue in Chicago. In order to support the gigantic structure, its civil engineering team needs to develop strong concrete foundation. The team recently synthesized a new concrete mixture which compressive strength is normally distributed with mu=30000psi and sigma=6000psi. Find the probability that a random sample of n=20 specimens will have a sample mean compressive strength that falls in the interval from 27000 to 31500 psi.