A thin cylindrical rod of length 10 ???? ???? 10cm is placed horizontally on the principle axis of a concave mirror of focal length 20 ???? ???? 20cm The rod is placed in a such a way that mid point of the rod is at 40 ???? ???? 40cm from the pole of mirror The length of the image formed by the mirror will be ???? 3 ???? ???? 3 x ? cm The value of ???? x is ______