Arsenic is a chemical element that occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. The main use of metallic arsenic is for strengthening alloys of copper and lead. Unfortunately, arsenic occurs in some ground water. In large quantities it is toxic for multicellular life. A mean arsenic level of 8 particles per billion (ppb) is considered safe for agricultural use. A well in Texas is used to water cotton crops. This well is tested regularly for arsenic. A random sample 37 tests gave a sample mean of 7.2 ppb arsenic with standard deviation of 1.9 ppb. Assume that arsenic levels are normally distributed. We are to use the sample data and test if mean arsenic level in the well in question is less than 8 ppb.