A block A of mass 1.6 kg kept on a horizontal surface is connected to one end of a horizontal spring whose other end is fixed to a rigid vertical wall. The spring and block A system is executing simple harmonic motion with an amplitude of 10 cm. Another block B of mass 0.9 kg is kept on the block A when block A is passing through its equilibrium position. The amplitude of  oscillation of the system of spring, block A and block B is