The county health department has concerns about the chlorine level at a local water park. The health department tests the water Monday with readings of 4 mg out of 100 mL. On Thursday when the water is retested, the health dept. gets readings of 5 mg out of 120 mL. Assume that the samples are independent and that they have been randomly selected. Construct a 90% confidence interval for the difference in the proportions of water samples on Monday and Thursday.