Answer
$\frac{1}{3}(x+y)$
Work Step by Step
The phrase 'One third of the sum of two numbers' is equivalent to adding one number to another number and then multiplying the result by $\frac{1}{3}$.
Therefore, representing the unknown numbers as $x$ and $y$, we can translate the phrase algebraically to $\frac{1}{3}(x+y)$.
