Question 70

A car travels a certain distance at 24 km/h and comes back at 36 km/h. Find the average speed for total journey.

Solution

Average speed of the journey is the harmonic mean of the speeds 24 and 36 km/hr

Harmonic mean of two numbers 'x' and 'y' = $$\frac{2}{\frac{1}{x} + \frac{1}{y}}$$

Average speed = $$\frac{2}{\frac{1}{24} + \frac{1}{36}}$$

= $$2 \div {\frac{36 + 24}{36 \times 24}} = \frac{2 \times 36 \times 24}{60}$$

= $$\frac{144}{5} = 28.8$$ km/hr

=> Ans - (A)

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