A rational number is defined as a number that can be expressed as the quotient of two integers, where the denominator is not zero.
Let's evaluate the options:
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7.8: This is a decimal number that can be expressed as \( \frac{78}{10} \), which is a rational number.
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88√: This notation seems incomplete or unclear, but if it implies \( 88 \cdot \sqrt{X} \) for some rational \( X \), it can potentially be irrational unless \( X \) is a perfect square.
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the fraction with numerator 8 and denominator square root of 8: This is \( \frac{8}{\sqrt{8}} \). Since \( \sqrt{8} \) is irrational, this fraction is not a rational number.
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7 - √7: The square root of 7 is irrational, hence \( 7 - \sqrt{7} \) is also irrational.
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π: Pi is a well-known irrational number.
Among these options, the only clearly rational number is 7.8.
So, the correct answer is 7.8.