The acceleration is given by:
[tex]a=\frac{v_f-v_0}{t}[/tex]In this case we know that the acceleration is -4 m/s2 and that the initial velocity is 30 m/s and the final one is zero. Plugging this values into the equation above and solving for t we have:
[tex]\begin{gathered} -4=\frac{0-30}{t} \\ t=\frac{-30}{-4} \\ t=7.5 \end{gathered}[/tex]Therefore, it takes 7.5 seconds for the car to stop.
