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We're asked to find the of the rock, given some mass and volume information.

We're given that the mass of the system before the rock was added was $120$ $"g"$. The mass after the rock was added was $250$ $"g"$, so the rock's mass is

$color(red)(m_"rock") = m_"final" - m_"initial" = 250color(white)(l)"g" - 120color(white)(l)"g" = color(red)(ul(130color(white)(l)"g"$

The initial volume of water in the graduated cylinder was $30$ $"mL"$, and the volume after the rock was added read $75$ $"mL"$, so the rock's volume is

$color(green)(V_"rock") = V_"final" - V_"initial" = 75color(white)(l)"mL" - 30color(white)(l)"mL" = color(green)(ul(45color(white)(l)"mL"$

The equation for the density of the rock is

$"density" = color(red)("mass")/color(green)("volume")$

And so we have

$color(blue)("density") = color(red)(130color(white)(l)"g")/color(green)(45color(white)(l)"mL") = color(blue)(ulbar(|stackrel(" ")(" "2.9color(white)(l)"g/mL" = 2.9color(white)(l)"g/cm"^3" ")|)$

rounded to $2$ .

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