To multiply \( 2.5 \) by \( -\frac{1}{3} \), you can perform the multiplication as follows:
\[ 2.5 \times -\frac{1}{3} = -\frac{2.5}{3} \]
To simplify \( -\frac{2.5}{3} \), you can convert \( 2.5 \) to a fraction.
\[ 2.5 = \frac{25}{10} = \frac{5}{2} \]
Now, replace \( 2.5 \) with \( \frac{5}{2} \):
\[ -\frac{2.5}{3} = -\frac{5/2}{3} = -\frac{5}{2} \times \frac{1}{3} = -\frac{5}{6} \]
Thus,
\[ 2.5 \times -\frac{1}{3} = -\frac{5}{6} \]
In decimal form, this is approximately \(-0.8333\). So the final answer is:
\[ -\frac{5}{6} \text{ or } -0.8333 \]