Question

on a 100km track, a train travels the first 30 km at a uniform speed of 30 km/h .how fats must the train next 70 km socas to average 40 km/h for the entire trip

Answer 1

time taken to travel 30km=1hour
$average \: speed = \frac{total \: distance}{total \: time}$
average speed =40kmph
distance left=70km
let time for second time = x

$40 = \frac{30 + 70}{1 + x} \\ 40 + 40x = 100 \\ 40x = 60 \\ x = \frac{60}{40} \\ x = 1.5$
so speed for second time =
$70 \div 1.5 \\ = 46.66$
so answer is 46.66kmph.