The speed of the rock, according to the gravitational acceleration is "v = g t". The time it takes to fall from given height "h = 12.4 m" is "t = \\sqrt{\\frac{2 h}{g}}"(which is found from the equation of motion "y(t) = h - \\frac{g t^2}{2}" by equating "y" to zero and solving for t).
Hence, substituting the time into the equation for speed, obtain "v = g t = g \\sqrt{\\frac{2 h}{g}} = \\sqrt{2 g h} \\approx 15.6 s".
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