To fill in the blank in the equation \( \frac{1}{6} = \frac{x}{12} \), you can find \( x \) by cross-multiplying:
\[ 1 \times 12 = 6 \times x \]
This simplifies to:
\[ 12 = 6x \]
Now, divide both sides by 6:
\[ x = \frac{12}{6} = 2 \]
So, the complete equation is:
\[ \frac{1}{6} = \frac{2}{12} \]
The blank can be filled with the number 2.