Notice that this is a proportion problem. The weight per pencil does not change as the number of pencils increases or decreases.

Set up equivalent ratios that compare the number of pencils to the weight of the pencils.

$\frac{20\;\text{pencils}}{12\;\text{ounces}} = \frac{50\; \text{pencils}}{\it x}$

Solve for $x$.

$x = 30$ ounces