To find the missing number that makes the fractions equal, we can set up the equation:
\[ \frac{1}{2} = \frac{x}{20} \]
To solve for \(x\), we can cross-multiply:
\[ 1 \cdot 20 = 2 \cdot x \]
This simplifies to:
\[ 20 = 2x \]
Now, divide both sides by 2:
\[ x = \frac{20}{2} = 10 \]
So, the missing number is:
\[ \boxed{10} \]