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Word Problems/Help Quadratic Equation Word Problem


A projectile is launched directly upwards and its height, H metres above the ground is given by H=20t-5t where t is the time in seconds. After how many seconds will the projectile be 20 metres about the ground?

When the height is 20, that means H = 20.
This gives the equation 20 = 20t - 5t.
Adding 5t - 20t to both sides gives 5t - 20t + 20 = 0.
Dividing by 5 gives t - 4t + 4 = 0.
Factoring gives (t-2)(t-2) = 0.

The solution is where t-2 = 0, or at t=2.
Putting that back in gives 20(2) - 5(2) = 40 - 5*4 - 40 - 20 = 20,
so that's the right answer.  

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Scott A Wilson


I have answered every question that was a story problems that had any relation to math for which an answer existed. I have ever answered some questions which had a vague relation to math, but still related.


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