What is the coefficient of $a^4b^2$ in the expansion of $\left(2a-\frac{b}{3}\right)^6$?  Express your answer as a common fraction.
<|im_start|>system
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<|im_start|>user
What is the coefficient of $a^4b^2$ in the expansion of $\left(2a-\frac{b}{3}\right)^6$?  Express your answer as a common fraction.
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
\frac{80}{3}
What is the coefficient of $a^4b^2$ in the expansion of $\left(2a-\frac{b}{3}\right)^6$?  Express your answer as a common fraction.
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