not by excel please

A manager has been using a certain forecasting technique to forecast demand for word processing units for the past five periods. Actual and forecasted amounts are shown below. He had difficulties in identifying whether the used technique is better than the weighted moving average (WMA) with 0.5 being the weight with the most resent, 0.3, 0.2, Calculate the total error for the WMA forecast? (Note: write the answer as a number with 2 decimal places only, e.g. 12.50,55.83 , 12.00)* Period 1 2 3 4 5 Demand 90 85 91 92 95 Forecasted 98 87 95 7 88