The accompanying data file contains salary data (in $1,000s) for 30 college-educated men with their respective BMI and a dummy variable that represents 1 for a white man and 0 otherwise. Model 1 predicts Salary using BMI and White as predictor variables. Model 2 includes BMI and White along with an interaction between the two. 

a. Use the holdout method to compare the predictability of the models 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 predictability of the models using k = 3.