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# The average speed of a car running at the rate of 15 km/hr during the first 30 km at 20 km/hr during the second 30 km and at 25 km/hr during the third 30 km is

$\begin {array} {1 1} (A)\;19.15\: km/hr & \quad (B)\;20\: km/hr\\ (C)\;19.65\: km/hr & \quad (D)\;19.75\: km/hr \end {array}$

Can you answer this question?

Average speed = Harmonic mean of the three speeds
$= \large\frac{ 3}{\large\frac{1}{15}+\large\frac{1}{20}+\large\frac{1}{25}}$
$= \large\frac{3}{\large\frac{20+15+12}{300}}$
$= \large\frac{3}{\large\frac{47}{300}}$
$= \large\frac{900}{47}$