Step 1: Let speed over C be $x$ km/hr
Speed for first two stretches combined = $4x$. Total distance = 6 km.
Step 2: Time for first two stretches
Distance = 4 km, speed = $4x \Rightarrow$ Time = $\frac{4}{4x} = \frac{1}{x}$ hr.
Step 3: Time for stretch C
Distance = 2 km, speed = $x \Rightarrow$ Time = $\frac{2}{x}$ hr.
Step 4: Average speed condition
Average speed for the race = $\frac{6}{(1/x) + (2/x)} = \frac{6}{(3/x)} = \frac{6x}{3} = 2x$. Given $2x = 20$, so $x = 10$ km/hr.
Correction: Wait, check calculation — since first two stretches total 4 km at $4x$, time = $\frac{4}{4x} = \frac{1}{x}$ hr; last stretch 2 km at $x$ → time = $\frac{2}{x}$ hr. Total time = $\frac{1}{x} + \frac{2}{x} = \frac{3}{x}$ hr. Average speed = $\frac{6}{(3/x)} = 2x$. Equate to 20 → $x = 10$. Hmm, but 10 is not in answer key? Let’s recheck — If $x=10$, first two stretches speed = 40 km/hr, time = $4/40 = 0.1$ hr = 6 min; last stretch = $2/10 = 0.2$ hr = 12 min. Total = 18 min → average speed = $6/(18/60) = 20$ km/hr. So $x=10$ is correct.
Therefore, Correct Answer is 10 km/hr.