Answer
60 feet/sec
Work Step by Step
$s(t) = 20t^\frac{3}{2}$
Average speed of the car from t=0 to t=9.
$(s(9)-s(0))/(9-0)=(20(9)^\frac{3}{2}-20(0)^\frac{3}{2})/(9-0)=540/9=60 feet/sec$
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