Inasmuch as I earn less than $2 in a day, there is every possibility to make $15 in a day and one of such places which does not require any investment is in; transcription and moderating job for some sites, with this, depending on the level at which you put to work, you can even earn more than that. I do not know the possibility of making that sum of dollars on only forums and even if you choose to work on all the forums, I doubt it is possible this is because most forums restrict the money that can be earned by users on a daily basis.
This type of target can be met and even beyond mostly on trading sites like; Forex and cryptocurrency trading. In those sites, it demands absolute skilled traders who have different strategies and technicalities. Traders there make even $500 in a day but there are also days where the odds are not in their favor. Through casinos which is a gambling platform, it is possible to make $15 but with this, one would only have to keep his fingers crossed because investment is involved and there is always a 50/50 chances of either winning or loosing.