I need to find the number of all possible strings in the following form:
$ (nxx)-nxx-xxxx$ , where $n =1...9 $and $x = 0...9$
My trying :
Actually I cant find any clue to solve this problem . This question came from previous year question of discrete mathematics .
Each $n$ can be chosen in 9 different ways and each $x$ can be chosen in 10 different ways. All choices are independent. So the total number of choices is:
$$(9\times10\times10)\times(9\times10\times10)\times(10\times10\times10\times10)=8.1\times10^{9}$$