By what percent would the value of Marci's stock need to fall for her to sell if it is currently worth $45 and she will sell at $30?

To determine the percentage decrease in the value of Marci's stock from its current worth to its selling price, we begin by calculating the decrease in value. The stock currently valued at $45 is set to be sold at $30, meaning the decrease is:

$45 - $30 = $15.

Next, we find the percentage decrease based on the original value, which is $45. The formula for calculating the percentage decrease is:

(Decrease in value / Original value) x 100.

Substituting in the numbers:

($15 / $45) x 100 = (1/3) x 100 = 33.33%.

This shows that the value of Marci's stock needs to fall by approximately 33 1/3% for her to sell it at $30. Thus, the correct answer is that the percentage decrease required is 33 1/3%.