If 300 jellybeans cost you x dollars. How many jellybeans can you purchase for 50 cents at the same rate?

$\frac{150}{x}$
If 300 jellybeans cost x dollars, for each dollar you get $\frac{300}{x}$ jellybeans.
Since each dollar has two 50 cents, you get $\frac{300}{2 x}$ ie $\frac{150}{x}$ jellybeans.