Question

on the 100 m track , a train travels  the first 30 km at the uniform  speed of 30 km/h. how fast must the train travel the next 70 km so as to average 40km/h for the entire trip?
 
 

Answer 1

for first 30 km 
IT would be v = s / t =>30 = 30 / t 
t = 1 hr
again for the next 70 km
let velocity be x
v = s / t
x = 70 / t
t = 70 / x
∴ total time taken is 1 + 70/ x = (70 + x) / x
total dist. travelled is given i.e. 100km
∴avg. velovity = total dist / total time
40 = 100 divided by (70 + x) / x
⇒40. (70+x) / x = 100
⇒2800 + 40x - 100x = 0
⇒ - 60x = - 2800
⇒ x = - 2800 / -60
⇒ x = 45.55 km/hr