$3.$ Identify the CustomerID for the customer who made the most purchases in the dataset $($Hint: Use the most appropriate measure of central tendency for the customer ID column$).$ Your answer should appear in cell F$19$

$4.$ Once you have determined which customer made the most purchases, determine how many invoices exist for this customer $($Hint: Use the COUNTIF function$).$ Your answer should appear in cell F$21.$