Assume that the Internet connection at your retail store is temporarily down and you need to quickly validate a customer's credit card number before sending it off for debit authorization. For our purposes, a credit card number is valid if it has $16$ digits and the sum of all digits is a multiple of $10,$ and invalid otherwise. Write a function validate$\_$card$($cardnum$)$ which takes a string of digits cardnum as input $($e$.$g$.\text{'}1111111111120133\text{'})$ and returns True if cardnum is a valid credit card number, and False otherwise.

You function should work like this: