Two cars A and B are moving in the same direction along a straight...

Velocity of car A, $v_A = 100$ km/h

Velocity of car B, $v_B = 80$ km/h

Velocity of the stone relative to car B is $v$. The velocity of the stone relative to the ground is $v_S = v_B + v = 80 + v$.

The velocity of the stone relative to car A is $v_{S/A} = v_S - v_A = (80 + v) - 100 = v - 20$.

The stone hits car A with a relative speed of $5$ m/s. Converting this speed to km/h:

$v_{S/A} = 5 \times \dfrac{18}{5} = 18$ km/h

Equating the relative speeds:

$v - 20 = 18 \Rightarrow v = 38$ km/h

Answer: $38$