To fill in the blanks for the equation \( \frac{1}{5} = \frac{?}{15} \), you can use cross-multiplication or set up a proportion.
Set up the proportion:
\[ 1 \times 15 = 5 \times ? \]
This simplifies to:
\[ 15 = 5 \times ? \]
Now, divide both sides by 5:
\[ ? = \frac{15}{5} = 3 \]
So, the completed equation is:
\[ \frac{1}{5} = \frac{3}{15} \]
Thus, the blank should be filled with 3.