Discussion

Explanation:

Let his average speed over the last stretch be x.

Hence, his average speed for first two stretches = 4x. So the total time taken to cover the three stretches $=\left(\frac{4}{4x}\right)+\left(\frac{2}{x}\right)$

His average speed over the race is 20 km/hr.

Hence, the time taken to complete the race = $\frac{6}{20}$

Hence, we have the equation $\frac{4}{4x}+\frac{2}{x}=\frac{6}{20}$ Solving this equation, we get x = 10 km/hr.

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