A new process has been developed that transforms ordinary iron into a kind of super iron called metallic glass. This new product is stronger than steel alloys and is much more corrosion-resistant than steel. However, it has a tendency to become brittle at high temperatures. It is desired to estimate the mean temperature at which it becomes brittle. A random sample of 20 pieces of metallic glass is selected. The temperature at which brittleness is first detected is recorded for each piece. The summary results give a sample mean X of 600 °C and a sample standard deviation s of 15 °C. Find a 90% confidence interval for the mean temperature at which metallic glass becomes brittle.