#### Answer

$ \frac{16b + 15}{24b^{3}}$

#### Work Step by Step

Given : $\frac{4}{6b^{2}} + \frac{5}{8b^{3}}$
$= \frac{2}{3b^{2}} + \frac{5}{8b^{3}}$
Now,
$3b^{2} = 3 \times b \times b$
$8b^{3} = 8 \times b \times b \times b $
Lowest Common Denominator, LCD = $24b^{3}$
This becomes : $\frac{2}{3b^{2}} \times \frac{8b}{8b} + \frac{5}{8b^{3}} \times \frac{3}{3}$
$= \frac{16b}{24b^{3}} + \frac{15}{24b^{3}}$
$= \frac{16b + 15}{24b^{3}}$