Answer: 240 ft/sec
Explanation:
If the distance s, of the car regarding his starting position, is defined as a function of time, by definition, the instantaneous velocity at any time, is given by the derivative of the displacement (distance from origin) respect the time, as follows;
v = ds / dt ⇒ v= d/dt (5 t³) = 15 t² = 15 t²
Replacing by t=4 s, we get:
v = 15. 4² = 15.16 = 240 ft/sec