Given,
volume of wooden block v0 = 1.6 m3
density of wood do = 800 kg/m3
density of water dl = 1000 kg/m3
wood has less density than water so it floats on it.
now, for floting condition,
vo do g = vl dl g
vl/vo = do/dl
vl / vo = 800 / 1000
vl / vo = 4 / 5
This is the fraction of wood inside water.
So, fraction of wood above water surface = 1 - 4 / 5
= 1/ 5
hence, 1/5 th of total volume remains above the water surface.