Answer
$x-\displaystyle \frac{1}{3}=\frac{1}{2}x$
Work Step by Step
$\left.\begin{array}{ccccc}
\text{ a number } & \text{decreased by } & \frac{1}{3} & \text{is} & \text{ half of that number } & \\
\downarrow & \downarrow & \downarrow & \downarrow & \downarrow & \\
x & - & \frac{1}{3} & = & \text{ half of x } & \\
\downarrow & \downarrow & \downarrow & \downarrow & \downarrow & \\
x & - & \frac{1}{3} & = & \frac{1}{2}x &
\end{array}\right.$
$x-\displaystyle \frac{1}{3}=\frac{1}{2}x$
