$\frac{100}{2}$

To simplify multiplication involving $50$, we relate it to a power of 10.

• $50$ is half of $100$, so it can be written as $\frac{100}{2}$. This allows for a quick calculation by dividing by $2$ and multiplying by $100$.
• $100-50$ is true but doesn't help with the multiplication simplification method described.
• $\frac{100}{50}$ equals $2$, not $50$.
• $\frac{1000}{20}$ equals $50$, but using $\frac{100}{2}$ is simpler and more aligned with the method.

Ok