Suppose

is the number of possible combinations for a suitcase with a lock consisting of

wheels. If you added one more wheel onto the lock, there would only be 9 allowed possible digits you can use for the new wheel. This means the number of possible combinations for

wheels, or

is given recursively by the formula

starting with

(because you can start the combination with any one of the ten available digits 0 through 9).
For example, if the combination for a 3-wheel lock is 282, then a 4-wheel lock can be any one of 2820, 2821, 2823, ..., 2829 (nine possibilities depending on the second-to-last digit).
By substitution, you have

This means a lock with 55 wheels will have

possible combinations (a number with 53 digits).
Answer:
D.
Step-by-step explanation:
Since this isn't a credit card, there are no interests or fees on a debit card, so A is incorrect. B is also incorrect. You only need identification when you are withdrawing or depositing at a bank, but purchases made in stores or online do not need your identification. You also don't need to record transactions in your checkbook (but it is recommend to keep track of purchases). Modern day technology already records transaction history and all you need to do is access it online.
D is correct because if someone steals your PIN for your debit card, they could go to stores and use that money. You can dispute charges and report to the bank if that happens.