Applicants for the University of Statland take two tests, one for writing ability and the other on critical thinking, and normalised scores are recorded on each between 0 and 1. For a given applicant, let X be the score on the writing test and Y the score on the critical thinking test. A model for the joint probability density function for the two test scores can be taken as: f(x, y)=⎧⎩⎨x+y0for 0