Every year, there are more than100 billion ATM transactions in the united states. Before each transaction, the card owner is required to enter his or her PIN. When an ATM card is issued, many financial institutions have the applicant select a 4-digit PIN. there are 10 possible choices for the first the first digit of the pin.10 possible choices for the second digit, and so on. What is the total number of possible choices of a PIN as an exponential expression.

• Social Security numbers consist of 9 digits.How many unique 9-digit numbers are there?
• Do you think the US will ever run out of Social Security numbers?
• What other everyday objects must use unique numbers, and how are their numbers determined? For example, how does a state government determine that each car has a unique set of license plates?