$\text{1/}$ ${40km/h=\dfrac{100}{9}m/s ; 60km/h=\dfrac{50}{3}m/s}$
$⇔a=\dfrac{v^{2}-v_{0}^{2}}{2s}=\dfrac{\dfrac{50}{3}^{2}-\dfrac{100}{9}^{2}}{2.1000}=\dfrac{25}{324}(m/s^{2}$
$\text{2/}$
$v=v_{0}+at$ $⇒20a+v_{0} (1)=0$
$s=v_{0}t+\dfrac{1}{2}at^{2}$
$120=20v_{0}+\dfrac{1}{2}.a.20^{2}$
$6=v_{0}+10a (2)$
$\text{Từ (1),(2)⇒}$ $\left\{\begin{array}{l}{20a+v_{0}=0}\\{10a+v_{0}=6}\end{array}\right.⇔\left\{\begin{array}{l}{a=-0,6(m/s^{2})}\\{v_{0}=12(m/s)}\end{array}\right.$