assume that the coffe and tea are substitutes . Derick and Thandi are vendors at the station. Derick sells tea and Thandi coffee. they usually sell this drinks at R5 per cup. However it is a cold monay morning and Thandi decides to decrease the proce of coffee for R4 a cup. the resulten effect will be movement along the demand curve for coffee takes place since it is a change in the price of coffee.