Two methods of measuring surface smoothness are used to evaluate a paper product. The measurements are recorded as deviations from the nominal surface smoothness in coded units. The joint probability distribution of the two measurements is a uniform distribution over the region 0  x  4, 0  y, and x  1  y  x  1. That is, fXY (x, y)  c for x and y in the region. Determine the value for c such that fXY (x, y) is a joint probability density function.