Let the cost of a white marble be \( w \) cents. Then, the cost of a blue marble would be \( w + 50 \) cents since it costs 50 cents more than a white marble.
According to the problem, Rocky buys 20 blue marbles for a total of $25. First, we need to convert $25 into cents:
\[ 25 \text{ dollars} = 2500 \text{ cents} \]
Now we set up the equation based on Rocky's purchase:
\[ 20 \times (w + 50) = 2500 \]
Expanding this equation:
\[ 20w + 1000 = 2500 \]
Next, we solve for \( w \) by subtracting 1000 from both sides:
\[ 20w = 2500 - 1000 \] \[ 20w = 1500 \]
Now, divide by 20:
\[ w = \frac{1500}{20} = 75 \]
Thus, the cost of the white marbles is \( \boxed{75 \text{ cents}} \).
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