answer 5-3: analysis of the ligand binding plots according to Y = [X] / (K + [X])
The analysis you requested yields the following result:
The Kd was 1.3 mM. We do not need the analysis of the residuals to see that the calculated line fits poorly the experimental points; moreover we have a logical inconsistency in this analysis because we realize that when [X]tot = 1.3 mM we do not have enough X to bind to half the available binding sites ([P]tot = 3 mM)! Clearly we are in a condition similar to the one described in tutorial #4 but more extreme. Thus we discard this analysis.
It is suggested that you go back one step and select a different answer.