A rational number is defined as a number that can be expressed as the quotient of two integers (i.e., it can be written in the form \( \frac{a}{b} \), where \( a \) and \( b \) are integers and \( b \neq 0 \)).
Let's analyze each option:
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\( 2\pi \): This is an irrational number because \(\pi\) is irrational and multiplying by 2 does not change its nature.
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\( \frac{6}{0} \): This expression is undefined because division by zero is not permissible.
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\( \sqrt{61} \): This is also an irrational number since 61 is not a perfect square.
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\( 0.37815 \): This is a decimal number that can be expressed as a fraction (e.g., \( \frac{37815}{100000} \)), which makes it a rational number.
Therefore, the rational number from the options provided is 0.37815.