Suppose that the price of muffins is 0.50 and the local cafeteria serves 800 muffins per day. At the same cafeteria, the price of coffee is increased from 0.75 to 1.25 per cup. No other changes have occurred and the number of muffins sold decreases to 600 per day. Are muffins and coffee complements or substitutes? Explain your answer with the elasticity concept.