Two cars A and B are travelling along a straight road in the same direction with velocities 20 km/h and 40 km/h, respectively; car A is ahead of car B. If the length of each car is 5 m, then the minimum length of the road required for car B to overtake car A is:

The left portion of the figure shows the situation when the process of overtaking has just begun. At this moment, the front P of car A and the rear Q of car B are separated by a distance of$5 m + 5 m =10 m$. The right portion of the flgure shows the situation when the process of overtaking has just been completed. The points P and Q are now placed alongside. Q has moved with velocity $40 km/h - 20 km/h = 20 kmlh$ relative to point P. Hence, the time taken to overtake is

$T=\frac{10\mathrm{m}}{20\mathrm{km}/\mathrm{h}}$

In this time, the distance travelled by point Q is 

$s=(40\mathrm{km}/\mathrm{h})T=20\mathrm{m}$

From the flgure, we can see that the total length of the road required for the process is

$s+5\mathrm{m}=25\mathrm{m}$