Answer: 250kg/m3
The duck condition would be 25% on water and 75% on air. That means the duck density is lower than the water and the duck is pushed up by the force. The force difference from 25% of duck volume can sustain 100% of the duck weight.
That mean : density of the duck/ density of water = 25%/100%= 0.25 x water density.
If the density of water is 1000kg/m3 then duck density : 0.25x 1000kg/m3= 250kg/m3
