To solve the expression \( \frac{1}{2} - \frac{7}{20} \), you first need a common denominator. The least common denominator (LCD) for 2 and 20 is 20.
Now, convert \( \frac{1}{2} \) to a fraction with a denominator of 20:
\[ \frac{1}{2} = \frac{1 \times 10}{2 \times 10} = \frac{10}{20} \]
Now, you can subtract \( \frac{7}{20} \) from \( \frac{10}{20} \):
\[ \frac{10}{20} - \frac{7}{20} = \frac{10 - 7}{20} = \frac{3}{20} \]
So, \( \frac{1}{2} - \frac{7}{20} = \frac{3}{20} \).