When a car travels a distance of d kilometers in time t hours, the average speed for the journey is given by a =4/t km h^-1. find: a. the average speed of a car which travels 200km in 2 1/2 hours. b. the distance travelled by a car in 3 1/4 hours if its average speed is 80 km h^-1. c. the time taken, to the nearest minute, for a car to travel 865 km at an average speed of 110 km h^-1