SOLUTION: An object is projected upward from the top of a tower. Its distance in feet above the ground after(t)seconds is given by s(t)= -16t^2+64t+80. How many seconds will it take to rea

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Question 82301: An object is projected upward from the top of a tower. Its distance in feet above the ground after(t)seconds is given by s(t)= -16t^2+64t+80. How many seconds will it take to reach ground level? The answer is multiple choice.
a. 1 second
b. 4 seconds
c. 5 seconds
d. 8 seconds
Answer by 303795(602) About Me  (Show Source):
You can put this solution on YOUR website!
s(t)= -16t^2+64t+80
When it reaches the ground the height will be zero so
0=-16t%5E2%2B64t%2B80
Divide each side of the equation 16 to simplify the numbers involved.
0=-t%5E2%2B4t%2B5
Multiply both sides of the equation by -1 to eliminate the negative sign in front of the t squared.
0=t%5E2-4t-5
Factorise the trinomial.
0=%28t-5%29%28t%2B1%29
ie t=5 or t=-1.
The first answer shows the object hits the ground after 5 seconds and the second shows that it was at ground level 1 second before it was thrown upwards.
In the context of the problem the 5 second answer must be the solution.