Answer
$x-\displaystyle \frac{1}{3}x$
Work Step by Step
$\left[\begin{array}{llll}
\text{ a number } & \text{decreased by } & \frac{1}{3}\text{ of itself } & \\
\downarrow & \downarrow & \downarrow & \\
x & - & \frac{1}{3}\text{ of x } & \\
\downarrow & \downarrow & \downarrow & \\
x & - & \frac{1}{3}x &
\end{array}\right]$
$x-\displaystyle \frac{1}{3}x$
