Answer
$\color{blue}{\frac{1}{2}(x+y)}$
Work Step by Step
The expression "sum of two numbers" means the operation involved is addition.
Thus, if we let the numbers be $x$ and $y$, then "the sum of two numbers" means $x+y$.
The expression "half of" means the operation involved is multiplication, and the number to be multiplied is $\frac{1}{2}$.
Therefore, "half the sum of two numbers" means:
$\color{blue}{\frac{1}{2}(x+y)}$
