find the density

You are having problem in calculating the buoyant force.What is the meaning of Buoyant force?It is the weight of the liquid displacedby the object.You should read buoyant force again.Now how to calculate the buoyant force?You can have volume of the sphere immersed in the liquid.Then it is equal to the volume of the water or liquid displaced.Multiply it with the density of the liquid and then by g and you have the buoyant force.This should be equal to the weight of the sphere as it is floating.The direction of the buoyant force is vertically upwards as it is the force due to the displaced liquid.
Therefore
10x2V/5g=mg=>m=4v
Density of the sphere=m/V=4
Please read the buoyant frce again.