To find the number that makes the ratio equivalent to 1:3 when combined with 5, we need to set up the equation based on the concept of equivalent ratios.
Let the unknown number be \( x \).
The given ratio is \( \frac{5}{x} \) and it should be equivalent to \( \frac{1}{3} \).
We can set up the equation:
\[ \frac{5}{x} = \frac{1}{3} \]
To solve for \( x \), we can cross-multiply:
\[ 5 \cdot 3 = 1 \cdot x \]
This simplifies to:
\[ 15 = x \]
Thus, the number that makes the ratio equivalent to 1:3 is \( \boxed{15} \).