Advanced MathVery hardTime a ball spends above a height
Advanced Math practice question
A ball's height h, in metres, t seconds after it is launched is given by h = −5t² + 30t. For how many seconds is the ball at or above a height of 40 metres?
Answer: 2
−5t² + 30t ≥ 40 gives t² − 6t + 8 ≤ 0, so (t − 2)(t − 4) ≤ 0 and 2 ≤ t ≤ 4. That is 2 seconds.
Common wrong answers
- If you answered 3
- 3 is the time of the maximum height.
- If you answered 4
- 4 is the time the ball drops back to 40 metres.
- If you answered 6
- 6 is the time the ball lands.
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