Question 15
Andy, Benny and Charlie will run on three straight parallel tracks $a,b,c$ (track $b$ is between tracks $a$ and $c$). At the start of the race, all three runners are on a line perpendicular to the three tracks. The distance between tracks $a$ and $b$ is twice the distance between tracks $b$ and $c$. During the whole race, the three runners will all run on a straight line. If the speeds of Andy and Charlie are 8 km/h and 20 km/h, respectively, what is the speed, in km/h, of Benny?