Answer :
The kinetic energy of an object with mass m and speed v is given by the expression:
[tex]K=\frac{1}{2}mv^2[/tex]Isolate v from the equation and substitute m=60kg and K=1.2x10^4J to find the speed of the student, which is the same as the speed of the car:
[tex]\begin{gathered} \Rightarrow v=\sqrt[]{\frac{2K}{m}} \\ =\sqrt[]{\frac{2(1.2\times10^4J)}{60kg}} \\ =\sqrt[]{\frac{24000\operatorname{kg}\cdot\frac{m^2}{s^2}}{60\operatorname{kg}}} \\ =\sqrt[]{400\cdot\frac{m^2}{s^2}} \\ =20\cdot\frac{m}{s} \end{gathered}[/tex]Use the conversion factor 1m/s=3.6km/h to write the speed in the requested units:
[tex]v=20\cdot\frac{m}{s}\times\frac{3.6\frac{\operatorname{km}}{h}}{1\frac{m}{s}}=72\frac{km}{h}[/tex]Therefore, the speedometer reading of the car in km/h is:
[tex]72[/tex]