Part A: Let's represent the average change in the temperature of the city each week as 'x'. Since the temperature changed by the same amount each week and decreased by a total of 15 degrees Celsius over 5 weeks, the expression for the average change in temperature per week can be written as: x = -15/5.
Part B: To simplify the expression, we perform the division: x = -15/5 = -3. This means that on average, the temperature of the city decreased by 3 degrees Celsius every week. The negative sign indicates a decrease in temperature. Therefore, the average change in the temperature of the city per week is a decrease of 3 degrees Celsius.