well, since we'll be assuming a year has 365 days, well, 105 days is really just 105/365 of a year, and since you paid back 600, that means the interest was 50 bucks, 600 - 550 = 50.
[tex]~~~~~~ \textit{Simple Interest Earned} \\\\ I = Prt\qquad \begin{cases} I=\textit{interest earned}\dotfill & \$50\\ P=\textit{original amount deposited}\dotfill & \$550\\ r=rate\to r\%\to \frac{r}{100}\\ t=years\to \frac{105}{365}\dotfill &\frac{21}{73} \end{cases} \\\\\\ 50 = (550)(\frac{r}{100})(\frac{21}{73})\implies 50=\cfrac{11550r}{7300}\implies 50=\cfrac{231r}{146} \\\\\\ 7300=231r\implies \cfrac{7300}{231}=r\implies \stackrel{\%}{31.6}\approx r[/tex]
