The vibration on the floor of a laboratory is found to be simple harmonic motion at a frequency in the range 15-60 Hz, (depending on the speed of some nearby reciprocating plant). It is desired to install in the laboratory a sensitive instrument which requires insulating from the floor vibration. The instrument is to be mounted on a small platform which is supported by three similar springs resting on the floor, arranged to carry equal loads; the motion is restrained to occur in a vertical direction only. The combined mass of the instrument and the platform is 40 kg: the mass of the springs can be neglected and the equivalent viscous damping coefficient of the suspension is 0.2. Calculate the maximum value for the spring stiffness, if the amplitude of the transmitted vibration is to be less than 10% of that of the floor vibration over the given frequency range.