Answer
$log_{7}5$
Work Step by Step
We know that $log_{b}\frac{x}{y}=log_{b}x-log_{b}y$ (where $x$, $y$, and $b$ are positive real numbers and $b\ne1$).
Therefore, $log_{7}20-log_{7}4=log_{7}\frac{20}{4}=log_{7}5$.
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