Current Attempt in Progress

An iron casting containing a number of cavities weighs $6\mathrm{.}66E+3N$ in air and $3\mathrm{.}59E+3N$ in water. What is the total volume of all the cavities in the casting? The density of iron $($that is$,$ a sample with no cavities$)$ is $7\mathrm{.}87\frac{g}{c}{m}^3.$

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