Question

a truck covers a distance of 150 km at a certain average speed and then covers another 200 km at an average speed which is 20 km per hour more than the first speed. if the truck covers the total distance in 5 hours, find the first speed of the truck.

Answer 1

time =5 hours 
let us take x as average speed to cover 150 km 
then x+20 will be the average speed to cover next 200 km
then
150/x+200/x+20=5
5 x square+100 x=350 x+3000
x square-50 x-600=0
by solving the equation 
we get the x=60 km
therefore average speed to cover 150 km=60 km/h

Answer 2

let the first speed be x
distance covered with this speed 150
another speed is x+20
dist covered is 200
t1=150/x
t2=200/x+20
t1+t2=5
150/x+200/x+20=5
hence we get following quadratic eq
x²-50x-600=0
on solving it we get
x=60
thus its speed in first case is 60km/h