<PRE><B>Question 763173</B>
I stand on a building 640 feet high and throw a rock into the air with a velocity of 96 ft/sec. When does the rock have a height of 100 ft? 
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Use the common function for height vs time on Earth,
{{{h(t) = -16t^2 + 96t + 640}}} h in feet, t in seconds
{{{h(t) = -16t^2 + 96t + 640 = 100}}}
{{{16t^2 - 96t - 540 = 0}}}
{{{4t^2 - 24t - 135 = 0}}}
*[invoke solve_quadratic_equation 4,-24,-135]
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Ignore the negative solution
t = 9.538 seconds
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