Let the triangle \( PQR \) be the image of the triangle with vertices \( (1, 3) \), \( (3, 1) \) and \( (2, 4) \) in the line \( x + 2y = 2 \). If the centroid of \( \triangle PQR \) is the point \( (\alpha, \beta) \), then \( 15(\alpha - \beta) \) is equal to:
