1010x1.65 10x10x10x10x10x10x10x10x10x10=10 billionx1.65=16500000000
The counting principle works like this.Multiply the number of choices you have for each character.If you can have 0000000000 then you would multiply 10x10x10x10x10x10x10x10x10x10 or 10,000,000,000.10 billion.If you eliminate that number that there is one less.
898 area codes x 889 exchanges x 10,000 numbers per exchange = 7,983,220,000 numbers.jk or: 10 billion possible combos if each number can be 0-9 then that's ten numbers. and ten numbers in a phone number so 10x10x10x10x10x10x10x10x10x10=10 billion
First position can be any one of 10 numbers. One number has been taken so the second position can be any one of 9 numbers. Third position can be any one of 8 numbers. 7,6,5,4,3,2... Last position can only be 1 number as all the others will have been taken. Which leaves you with 10x9x8x7x6x5x4x3x2x1 or 10! (factorial). 10! = 3,628,800 combinations. If numbers can be repeated this makes things easier as you can just multiply by 10 for each position - 10x10x10x10x10x10x10x10x10x10 or 10^10. 10^10 = 10,000,000,000 combinations.