A car moves a distance of 200 m . It covers first half of the distance at speed 60kmh-1 and the second half at speed v. If the average speed is 40kmh-1, the value of v is

Time taken by the car to cover first half of the distance is t1=100/60. Time taken by the car to cover speed half of the distance is t2=100/v. vav=(100+100)/(t1+t2)⇒40=200/(100/60+100/v).