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Can someone help with the following 2 questions?

1. An iron casting containing a number of cavities weighs 6000 N in air and 4000 N in water. What's the total volume of all cavities in casting? The density of iron without cavities is 7.87 g/cm^3. The density of water is 988 kg/m^3.

Wouldn't the bouyant force = the mass of displaced water * g. So here, bouyant force = 6000 - 4000 = 2000. So wouldn't mass of displaced water = 2000/9.8. And doesn't this need to equal to the mass of the cavities? There seems to be something I'm missing.

2. The volume of air space in the passenger compartment of an 1800 kg car is 5 m^3. The volume of motor and front wheels is 0.750 m^3. The volume of the rear wheels, gas tank and trunk is .8 m^3. Water can't enter into the last 2 compartments (only the passenger compartment can water enter). As water enters, the car sinks. How many cubic meters of water are in the car as the car sinks?

To sink, the force must be greater than mg (1800*g). But I don't see the significance of the point of sinking? Since water can't enter into the rear wheels and front wheels, does this mean that there's an upward force equal to (.750+.8)*density of air*g? If anyone has any hints, that would be great.

Thanks.