A ball is thrown into the air. A sensor measures the height of the ball above the ground. The height of the ball is described by the equation y=-5x(x-4), where x represents the time in seconds. Determine the intervals of time when the ball is above a height of 15 m.

Question

whpalmer4 · Answer

$$y = -5x(x-4) = -5x^2+20x$$Find the time where $y>15$
$$15 = -5x^2+20x$$Solve for $x$.  You'll get two solutions, and the time between the two (not including the solutions!) will be the interval of time where the ball is above 15 m.