Bài giải :

$B=\frac{36}{1.3.5}+\frac{36}{3.5.7}+\frac{36}{5.7.9}+...+\frac{36}{25.27.29}$

$B=9\left( \frac{4}{1.3.5}+\frac{4}{3.5.7}+\frac{4}{5.7.9}+...+\frac{4}{25.27.29} \right)$

$B=9\left( \frac{1}{1.3}-\frac{1}{3.5}+\frac{1}{3.5}-\frac{1}{5.7}+...+\frac{1}{25.27}-\frac{1}{27.29} \right)$

$B=9\left( \frac{1}{1.3}-\frac{1}{27.29} \right)$

$B=9\left( \frac{1}{3}-\frac{1}{783} \right)$

$B=\frac{260}{87}$

Vậy B<3.