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CA Foundation · Quantitative Aptitude · Equations

A cricket ball is thrown upward so that its height in metres after t seconds is h = 30t − 5t². After how many seconds does the ball return to the ground?

The ball returns to the ground after 6 seconds. Setting height 30t−5t² equal to zero gives 5t(6−t)=0, so t=0 at launch or t=6 on return. The 3-second value is only the time of maximum height, halfway through the flight.

  1. A3 seconds
  2. B8 seconds
  3. C6 secondsCorrect
  4. D12 seconds

Explanation

The ball is on the ground when h=0: 30t−5t²=0, so 5t(6−t)=0, giving t=0 or t=6. The return is at t=6 seconds. Check: h(6)=180−180=0. The value 3 is when the ball reaches maximum height, the midpoint.

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