Question

a car moving on a straight road covers one-third of the total distance with 20 km/h and the remaining distance with 60 km/hour. What is the average speed of the car?

Answer 1

Average speed is the ratio of total distance travelled to total time taken.

So if X is the total distance travelled then we have given that X/3 distance covered with speed 5.5 m/s (20 km/h) and 2X/3 distance covered with speed 16.67 m/s (60km/s).

So time taken to cover X/3 distance= (X/3)/5.5=X/16.5 seconds.

And time taken to cover 2X/3 distance=

(2X/3)/16.67=2X/50.01 seconds.

So total time taken to cover distance X = (X/16.5)+(2X/50.01)=0.1X seconds.

And hence average speed= X/0.1X =10m/s =36km/h.

(Formula for time calculation:

Time=distance/speed)

Answer 2

let the total distance be  3x

time taken to cover 1/3 of the total distance = x/20

time taken to cover 2/3 of the total distance = 2x/60 = x/30

total time = x/20 + x/30 = 5x/60 = x/12

average speed = (total distance travelled) / total time taken (3x)/(x/12) = 36 km/hr

 hope it helps
cheers!