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A bus is traveling at 80feet per second. suddenly, the driver appies the brakes. The speed of the braking bus after t seconds is given by f=80-20t^2.

How long will it take the bus to come to a complete stop?  I do not know how to write this out or iam missing something. the answer is 2 sec. but I can not get that. please help me see where I am off. thank you

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Kirill Z. | Physics, math tutor with great knowledge and teaching skillsPhysics, math tutor with great knowledge...
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You need to find when the speed is zero, this would mean the bus comes to a complete stop. Since the speed after braking is given by:
setting this equal to zero yields:
80-20t2=0 Or 80=20t2, which leads to 
t2=80/20=4, from which it is easy to see that
t=2 seconds