A and B go cycling in the same direction with speeds of 6 km/hr and 12 km/hr. A car from behind passes them in 9 and 10 seconds respectively. What is the speed of the car?

A. 22 km/hr
B. 33 km/hr
C. 66 km/hr
D. 44 km/hr
The relative speed of A and B is 6 km/hr = 1.67 m/s
As the car passes A after 10s, the distance between A and B after 10s (i.e. at 11th second) is the distance covered by car in 1 second.
Therefore, at t = 11, d = 1.67 * 11
d = 18.33 m
v = d/t = 18.33/1 = 18.33m/s
v = 66 km/hr