Let the density of bob is σ & density of liquid is 16σ .
Now, Net downward force on bob = Weight - Buoyancy
=σVg−16σVg=1615σVg
Net effective gravitational acceleration =σV1615σVg=1615g
Time period (T)=2πgeffl=2π1615gl=2π1516gl=1516⋅(2πgl)=154T
=4T151