Question

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Answer 1

1/3 of the volume of the wood block displaces a mass of water equal to the mass of the wood: 

Mwood = Mwater 
Vwood = 3Vwater 

To completely submerge the wood, you would therefore have to add enough weight to displace the entire mass of wood, which is three times the total mass of the wood. (You know this, because just 1/3 the volume of the wood displaced water equal to the entire mass, so three times that mass would displace enough water for the whole block of wood.) 

So add two times the mass of the wood. Since the wood has 6 kg of mass, add 12 kg more