Relative speed of train to person = (60-6) km/hr = 54, equates to 54×5/18 = 15 m/s. Train crosses 15×12 = 180 m length. Train pass platform 180 m in 20 seconds, platform length is (60×5/18 = 16.67 m/s). Platform length = speed × time - train length, ~153.33 m.