Expand the following:
$\left(\frac{1}{x}+\frac{y}{3}\right)^{3}$

We use the algebraic identity: $(a+b)^{3} = a^{3} + b^{3} + 3ab(a+b)$.
Here,$a = \frac{1}{x}$ and $b = \frac{y}{3}$.
Substituting these values into the identity:
$\left(\frac{1}{x} + \frac{y}{3}\right)^{3} = \left(\frac{1}{x}\right)^{3} + \left(\frac{y}{3}\right)^{3} + 3 \left(\frac{1}{x}\right) \left(\frac{y}{3}\right) \left(\frac{1}{x} + \frac{y}{3}\right)$
$= \frac{1}{x^{3}} + \frac{y^{3}}{27} + \frac{y}{x} \left(\frac{1}{x} + \frac{y}{3}\right)$
$= \frac{1}{x^{3}} + \frac{y^{3}}{27} + \frac{y}{x^{2}} + \frac{y^{2}}{3x}$
Rearranging the terms,we get: $\frac{1}{x^{3}} + \frac{y}{x^{2}} + \frac{y^{2}}{3x} + \frac{y^{3}}{27}$.