Plot a graph of distance against time for the motor cars. Determine from the graph how many motor cars are met by a passenger riding in a motor car starting from point $B$ to point $A$ simultaneously with one of the motor cars from point $A$. The car in which the passenger is riding travels at a speed of $60\ \mathrm{\displaystyle \frac{km}{h}}$.

Eleven motor-cars.

     

Assume that the passenger has been travelling from $B$ for one hour. On the graph, the motion of the motor‑car with the passenger is represented by line $BC$. Lines 2, 3, 4, etc represent the motion of the motor‑cars from $A$ which started out 50, 40, 30, etc. minutes before the departure of the passenger. Lines 8, 9, 10, etc. represent the motion of the motor‑cars from $A$ which started out 10, 20, 30, etc. minutes after the deparature of the passenger. It is obvious that the number of motor‑cars passed by the passenger is equal to the number of points of intersection of these lines with line $BC$.