A prestressed concrete beam of cross-sectional area $A$, moment of inertial ${}^{\prime }{l}^{\prime }$, distance of top extreme fibre from neutral axis ${}^{\prime }{y}^{\prime }$ and distance of bottom extreme fibre from neutral axis ${}^{\prime }{y}_b^{\prime }$ is subjected to a prestressing force such that stress at top fibre is zero. What is the value of eccentricity ('r' is radius of gyration)?