# How do you calculate Log_7 15 ?

${\log}_{7} 15 = 1.39167$
As ${\log}_{a} b \times {\log}_{b} c = {\log}_{a} c$
${\log}_{b} c = {\log}_{a} \frac{c}{\log} _ a b$
Hence ${\log}_{7} 15 = \log \frac{15}{\log} 7 = \frac{1.1761}{0.8451} = 1.39167$