Cylinder is in equilibrium so weight = upthrust. Let entire volume of cylinder be Vc this means mass of cylinder = density of wood * volume of cylinder = 600Vc.
Upthrust = density of water * volume of water displaced( equal to volume of cylinder under water, let this equal Vw) * g = 1000 * Vw * g.
As it is in equilibrium : 600Vc * g = 1000 Vw* g
600Vc = 1000Vw
Vw/Vc=600/1000 = 0.6