How to reach specific location in C#?

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C#

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Hi, I have a scanned image which contains some multiple-choice questions. So, first of all how could I calculate the average color for most pixels inside a specific circle (I already know how to get the color of one individual pixel I only need help with the algorithm)
Please Note I already worked on those scanned images and made every scanned photo 100% similar to the other. (the only difference is dpi)

Secondly, how could I reach an constant location, at first I thought of reaching pixels but what if my scanned image has 150dpi, and the same image was scanned with 300dpi since this will change the x and y values for that pixel?

What I have tried:

I already explained that. Sorry for the inconvenience