Answer
$x=\displaystyle \frac{1}{3}\log 1.05$
Work Step by Step
The base $b$ logarithm of $x,\log_{b}x,$
is the power to which we need to raise $b$ in order to get $x$ .
Symbolically, $\quad \log_{b}x=y\quad $ means $\quad b^{y}=x$
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$300(10^{3x})=315\qquad/\div 300$
$ 10^{3x}=1.05\qquad$... which means $\ \ 3x=\log 1.05$
Divide the last equation with 3,
$x=\displaystyle \frac{1}{3}\log 1.05$
