If I use
$15 in the editor sometimes it's showing weird characters. here, I removed the $ to make my answer readable, but if you have high rep you can roll back the edit and I guess you could see it.
Those are formatting symbols on many Stack Exchange sites.
$ begins a section of $\LaTeX$ code, which doesn't work here on meta.cogsci, but it works on the main site. ` begins a section of code-formated text. I've used it on the preceding $ sign, but since it's active here (and on every SE site I've tried it on so far), I won't use it again in this answer. If I did, everything since my first use of the grave accent symbol would appear in code formatting. I've embedded your $ signs in code-formatted sections to prevent them from opening sections of LaTeX code, but if you prefer, you can produce a simple $ with LaTeX following @ChuckSherrington's advice by entering $\$$.
...Actually, I see his advice was even better: just enter \$ for each one!