+614 A city has to accommodate for 9.99 million households requiring a telephone number. They are about to issue 8-digit telephone numbers, with 5 as the first digit.
How many different telephone numbers can be created if repetition of digits is not allowed?
+538 181440
The First digit is 5 as said.
Each digit after that has to be different.
There are 10 digits, so we multiply 1 less each time we move to the right.
The second digit has 9 possibilities - any digit other than 5.
The third has 8 - Any except 5 and the first digit.
And so on.
So 1*9*8*7*6*5*4*3=181440
+538 181440
The First digit is 5 as said.
Each digit after that has to be different.
There are 10 digits, so we multiply 1 less each time we move to the right.
The second digit has 9 possibilities - any digit other than 5.
The third has 8 - Any except 5 and the first digit.
And so on.
So 1*9*8*7*6*5*4*3=181440
