Petrol is poured into the tank of a car at a constant rate. Initially there are 12 litres of petrol in the tank. After 20 seconds there are 40 litres of petrol in the tank.
(a) Draw a graph to show this.
(b) Use your graph to find the amount of petrol in the tank after 15 seconds.
(c) Work out the rate of filling the tank.
(d) There are y litres in the tank after x seconds. Write an equation for the graph.