We don't know the significance of the numbers in the equation, but we can use the fact that it relates distance and time, just as speed does. We know that the distance must be at least 50 and no more than 53, so we can find t for these two values of distance. If S=3t+47 then t=(S-47)/3. t1=(50-47)/3=1 and t2=(53-47)/3=2. So the minimum t is t1=1 and the maximum is t2=2.
If you're still puzzled about the equation, think of it like this: the first 47km doesn't count in determining the winner, and it doesn't matter what speed the driver drives his vehicle over this distance. All the important stuff takes place after 47km. It may be how long the driver can perform some stunt, like driving the car on two wheels only at a certain speed (e.g., 3kph, 3 metres per second, 3 lengths or laps per minute), or it may be an obstacle course of some sort, or steep gradients, etc. Nor do we know what the units of time are: they could be seconds, minutes, hours or some other intervals of time.