I4.2 Given that $b=\frac{\log 8^{a}+\log 27^{a}+\log 125^{a}}{\log 9+\log 25+\log 2-\log 15}$, find the value of $b$.
$b=\frac{a \log 8+a \log 27+a \log 125}{2 \log 3+2 \log 5+\log 2-\log 3-\log 5}=\frac{a(3 \log 2+3 \log 3+3 \log 5)}{\log 3+\log 5+\log 2}=\frac{2}{3} \times 3=2$