The 100-lb platform Ais attached to springs B and D, each of which has a constant k = 120...

The 100-lb platform Ais attached to springs B and D, each of which has a constant k = 120 lb/ft. Knowing that the frequency of vibration of the platform is to remain unchanged when an 80-lb block is placed on it and a third spring Cis added between springs B and D,determine the required constant of spring C.