Question 78

A car travels a certain distance at 70 km/h and comes back at 30 km/h. Find the average speed for total journey.

Solution

Average speed of the journey is the harmonic mean of the speeds 70 and 30 km/hr

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

Average speed = $$\frac{2}{\frac{1}{70} + \frac{1}{30}}$$

= $$2 \div {\frac{30 + 70}{30 \times 70}} = \frac{2 \times 30 \times 70}{100}$$

= $$2 \times 3 \times 7 = 42$$ km/hr

=> Ans - (A)

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