How do I politely tell my manager I won’t be studying flash cards off the clock?

I recently started working with insurance at a medical office. In some instances it’s helpful to know the procedure codes off the top of my head. During an eval today when asked “what can I improve on?” I said “i don’t know, I guess it’d be helpful to know more of the procedure codes but im sure I’ll become familiar with it over time.”

My boss then suggested 30 flash cards over 30 days, and I am almost certain I will be expected to do this in my personal time.

I respect my time too much to do this. How do I politely tell them I won’t be working off the clock?

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