4. A toy rocket is launched from the top of a cliff with an initial vertical velocity of 64 ft/s. The height h (in feet) of the rocket as a function of time t (in seconds) is modeled by the
equation:
h(t)=–16t^2+64t+80

answer:  

\boxed {\frac{1}{5}}  

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solution-

60 *   \frac{1}{5}   = 12

90 * \frac{1}{5} = 18

75 * \frac{1}{5} = 15

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hence, the answer would be \boxed {\frac{1}{5}}