Engineering Board Exam Reviewer · Quadratic Equations · medium

A ball is thrown upward and its height is h=40t-5t2 meters after t seconds. At what time is it 60 m high while coming down?

1. 2 s
2. 4 s
3. 8 s
4. 6 s

Answer

D. 6 s

Solution

Set 40t-5t2=60 t2-8t+12=0 (t-2)(t-6)=0. The ball is at 60 m at t=2 s (going up) and t=6 s (coming down).

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