Problema Solution
A bus from Pretoria to Johannesburg averages 80km per hour. On the trip from Johannesburg to Pretoria it averages 100km per hour. What is the average speed of the bus for the complete round trip?
Answer provided by our tutors
let
d =the distance from Pretoria to Johannesburg
t1 = the time of the travel from Pretoria to Johannesburg
t2 = the time of the travel from Johannesburg to Pretoria
d/t1 = 80
d = 80t1
d/t2 = 100
d = 100t2
that is
80t1 = 100t2
t1 = (100/80)t2
t1 = (5/4)t2
the average speed of the bus for the complete round trip is
2d/(t1 + t2) = 2d/(t2((5/4)+1) = 2(d/t2)*(4/9) = 2*100*(4/9) = 88.89 km per hour.