Simple interest formula:
I = Prt
where I is the interest, P is the principal, r is the interest rate (as a decimal), and t is time
Substituting with P = $300, r = 0.07, and i = $105, we get
[tex]\begin{gathered} 105=300\cdot0.07\cdot t \\ \frac{105}{300\cdot0.07}=t \\ 5=t \end{gathered}[/tex]