A rational number is a number that can be expressed as the quotient of two integers (a fraction \( \frac{a}{b} \)), where \( b \) is not zero.
Let's evaluate the options provided:
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\( 2\pi \): This is an irrational number because \( \pi \) is irrational, and multiplying by a rational number (2) does not change that.
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\( \frac{6}{0} \): This is undefined because division by zero is not allowed. Therefore, it's not a rational number.
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\( \sqrt{61} \): This is also an irrational number because 61 is not a perfect square.
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\( 0.37815 \): This is a decimal number. It can be expressed as a fraction \( \frac{37815}{100000} \), which is a ratio of two integers. Hence, it is a rational number.
Considering all the options, the rational number is 0.37815.