A car covers four successive $6km$ stretches at speeds of $25kmph$, $50kmph$, $75kmph$ and $150kmph$ respectively. Its average speed over this distance is:

Since the distances are equal, we use the harmonic mean formula for average speed: 4 / (1/v1 + 1/v2 + 1/v3 + 1/v4). Substituting the given speeds, we calculate 4 / (1/25 + 1/50 + 1/75 + 1/150). Finding a common denominator of 150, this becomes 4 / ((6 + 3 + 2 + 1)/150), which is 4 / (12/150). This simplifies to 4 * 150/12, resulting in an average speed of 50 kmph.

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