Recall that the cross price elasticity of demand is the ratio of the percentage change in quantity of one good to the percentage change in price of the other. The change in price of broccoli is from $2 per head to $2.10 per head, an increase of 5%. At the same time sales of cauliflower fell from 100 to 90 heads per week, representing a decrease of 10%. Thus the cross price elasticity is -2. Since an increase in price of broccoli was associated with a decrease in sales of cauliflower they are complements.