Question

A car covers three successive 4km distances at the speed of 10kmph,20kmph and 60kmph respectively its average speed for the total distace

Answer 1

In First journey,
Distance=4km
Speed=10kmph
$time = \frac{distance}{speed } = \frac{4}{10} = \frac{2}{5}$
In second journey,
Distance=4km
Speed=20kmph
$time = \frac{distance}{speed} = \frac{4}{20} = \frac{1}{5}$
In third journey,
$time = \frac{distance}{speed} = \frac{4}{60} = \frac{1}{15}$
In whole journey,
Total distance=4+4+4=12km
Total time--------
$\frac{2}{5} + \frac{1}{5} + \frac{1}{15} \\ = \frac{6 + 3 + + 1}{15} = \frac{10}{15} = \frac{2}{3}$
$average \: speed = \frac{total \: distance}{total \: time} \\ = \frac{12}{ \frac{2}{3} } = 12 \times \frac{3}{2} = 6 \times 3 \\ = 18kmph$
So, average speed is 18km/hr.