Answer: 12.6 s
We would use the equation of motion:
[tex]v-u=at[/tex]
where v is the final velocity, u is the initial velocity, a is the acceleration and t is the time taken.
It is given that the lead car has an initial velocity, [tex]u=24.7\hspace{1mm} m/s[/tex]. It decelerates with [tex]a=-1.96 \hspace{1mm} m/s^{2} [/tex] to stop (v=0). we need to calculate the time taken by the lead car to stop.
[tex]\Rightarrow t=\frac{v-u}{a}\\ \Rightarrow t=\frac{0-24.7}{-1.96}=12.6\hspace{1mm} s[/tex]