Let's start by figuring out how many students were cast in total and how many of them had major roles.
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Total students who auditioned: 90
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Fraction who were cast: \( \frac{1}{3} \)
Number of students cast: \[ \text{Number of cast students} = \frac{1}{3} \times 90 = 30 \]
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Number of students with major roles: 6
Next, we need to find the fraction of those who were cast that received major roles:
\[ \text{Fraction with major roles} = \frac{\text{Number of students with major roles}}{\text{Total number of cast students}} = \frac{6}{30} \]
Simplifying that fraction: \[ \frac{6}{30} = \frac{1}{5} \]
To express this as a decimal: \[ \frac{1}{5} = 0.2 \]
Thus, the fraction of those who were cast that got major roles, expressed as a decimal, is: \[ \boxed{0.2} \]