The trainee travels by means of a scooter and is only able to depart his home after sunrise (07:00), due to broken lights . The distance from his house to the training site is 60 km. If he used a car, still departing 07:00, he would travel an average of 30 km/h faster and reach the training site 20 minutes earlier. Take the average speed of the scooter as s. Use the following variables for each quantity: time = t; average speed = s; distance = d.

Determine:
an expression for the average speed of the car