To solve the equation \( x^{5/4} = 243 \), we first isolate \( x \):
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Rewrite the equation in terms of \( x \): \[ x = 243^{4/5} \]
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Calculate \( 243 \): \[ 243 = 3^5 \]
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Substitute \( 3^5 \) into the equation: \[ x = (3^5)^{4/5} \]
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Use the power of a power property: \[ x = 3^{5 \cdot (4/5)} = 3^4 \]
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Calculate \( 3^4 \): \[ 3^4 = 81 \]
Thus, the correct answer is: \[ \boxed{81} \]