Answer
$\therefore \log _{b} 4=-6$
Not possible, log values cannot be
negative.
Work Step by Step
Given $\log _{b} 5=10, \log _{b} 20=4$
$\log _{b} 4=?$
Using
$\log _{b}\left(\frac{m}{n}\right)=\log _{b} m-\log _{b} $
Put $m=20, n=5$
$\quad \therefore \log _{b}\left(\frac{20}{5}\right)=\log _{b} 20-\log _{b} 5$
$\Rightarrow \log _{b} 4=4-10$
$\therefore \log _{b} 4=-6$
Not possible, log values cannot be
negative.