The table below shows the platinum concentrations measured in plasma from blood samples collected from a patient receiving a platinum compound for the treatment of a tumour. The data are the concentrations at 4 hour intervals in the 24 hour period immediately after treatment and at 48 and 72 hours. Time (hours) 4 8 12 16 20 24 48 72 Platinum conc. (g/ml plasma) 2.70 2.40 2.34 2.60 2.41 2.20 1.82 1.58 It is thought that the level of platinum Y decreases with time X so that Y = bX + a, where Y is the platinum concentration. Fit the linear relationship to this data set and estimate the time at which the platinum concentration dropped to 1.75 g/ml.