Question

How many area codes of the form (XYZ) are possible if the digit 'X' and 'Y' can be any number ( through 9 but they can't repeat and the digit 7 can be any number 1 through 9?

Answer 1

Start to see the possible options

`XYZ=-\cdot-\cdot-_{}`

The first digit will have 10 possible numbers to choose from 0 to 9, however in the second digit since it cannot repeat there will be only 9 possible to choose from. As for ther third number 0 is not an option meaning that there are 9 to choose as well.

`\begin{gathered} XYZ=10\cdot9\cdot9 \\ XYZ=810 \end{gathered}`