A car staring from rest acceleration at the rate $f$ through a distance $S$, then continues at constant speed for time $t$ and then decelerates at rate $\large\frac{f}{2}$ to come to rest. If the total distance covered is $15\;s$ , then

Question

$\begin{array}{1 1} s= \large\frac{ft^2}{72} \\ s= \large\frac{ft^2}{4} \\ s= \large\frac{ft^2}{6} \\s= \large\frac{ft^2}{2} \end{array}$