Consider the accompanying data to predict an Under Armour purchase (Purchase; 1 for purchase, 0 otherwise) on the basis of customer age. 

a. Use the holdout method to compare the accuracy rates of the linear probability model (Model 1) and the logistic regression model (Model 2) using the first 20 observations for training and the remaining 10 observations for validation. 

b. Use the k-fold cross-validation method to compare the accuracy of the models using k = 3.