Question

On a 120km track, a train travels the first 30 km at a uniform speed of 30km/hr. How fast must the train travel the next 90 km so as to average 60 km/hr for the entire trip?

Answer 1

$avg \: velocity = \frac{total \: distance}{total \: time} \\ \frac{30 + 90}{1 + x} = 60 {kmhr}^{ - 1} \\ x = 1hr \\ so \: the \: next \: 90km \: should \: be \: travelled \: in \: 90 {kmhr}^{ - 1} to \: get \: a \: avg \: velocity \: of \: {90kmhr}^{ - 1}$