A simple pendulum with as solid metal bob has a period T.What will be the period of the same pendulum if it is made to oscillate in a non viscous liquid of density one tength of the metal of the bob?  

When we calculate the time period of simple pendulum then restoring force is considered to be due to the weight of the body. In air it is mg. In viscous liquid there will be upthrust on the bob. So the restoring force will be the effective weight = true wt- upthrust. To calculate upthrust you put it  equal to weight of equal volume of water. With these hints you should be able to calculate.