(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known dataGiven that the ASCII character system has 128 possible characters how many 5 character strings are there with at least one occurence of the '@' symbol.

2. Relevant equations

3. The attempt at a solution

So clearly which symbol we're using doesn't matter, and I see that the right answer comes from considering the total number of length 5 character strings minus the number of strings not containing the '@' character i.e. [tex]128^5-127^5[/tex].

My real question is what is wrong with the reasoning that we have 5 choices for where to put the '@' symbol multiplied by the [tex]128^4[/tex] possible strings from the other 4 characters. I can see that [tex]128^5-127^5 \not= 5(128^4)[/tex], but I'm wondering where the flaw in the reasoning is

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Counting strings

**Physics Forums | Science Articles, Homework Help, Discussion**