Mohit goes to his office at the speed of 10 m/s and returns to his home at the speed of x km/hr. If average speed of Mohit for the whole journey is 12 m/s, then what is the value of x?
Average speed = $$\dfrac{2ab}{a+b}$$ where a and b are speeds.
Here, a = 10 m/sec and b = x m/sec
Average speed = $$\dfrac{2\times10\times x}{10+x} = \dfrac{20x}{10+x}$$
Given, Average speed = 12 m/sec
$$\dfrac{20x}{10+x} = 12$$
=> $$20x = 120+12x$$
=> $$8x = 120 => x = 15$$
Therefore, $$x = 15 m/sec = 15\times\dfrac{18}{5} = 54 km/hr$$
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