Final answer:
In 5-Fold cross validation, each training set would have a size of 4,000. The estimated error for 4-NN classification using 5-Fold cross validation can be calculated with the provided summary of errors.
Step-by-step explanation:
(a). To find the size of each training set used for 5-Fold cross validation, divide the total size of the data set (20,000) by the number of folds (5). Each training set would have a size of 4,000.
(b). To estimate the error for 4-NN classification using 5-Fold cross validation, the summary of errors is needed.