The accompanying data file contains daily sales (in $) at Caf Venetian for 100 days. For cross-validation,

The accompanying data file contains daily sales (in $) at Café Venetian for 100 days. For cross-validation, let the training and the validation sets comprise the first 80 days and the last 20 days, respectively. 

a. Use the training set to estimate the linear and the exponential trend models and compute the resulting MSE, MAD, and MAPE for the validation set. 

b. Determine the preferred model and reestimate it with the entire data set to forecast sale for the 101st day.