Question: A thief walks up to an electronic lock with a 10-digit keypad and he noticed that all but three of the keys are covered in dust while the 2, 4 , 6 and 8 keys show considerable wear.
He thus can safely assume that the 4-digt code that opens the door must be made up of this number in some order.
Problem: What is the worst case number of combinations he must now test to try open this using a brute-force attack?
Can you answer this problem using basic computer networking concepts and securities.