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You have to break your problem down into smaller steps. Separate the user input from the arithmetic part.

Since you're stuck on the arithmetic part, then simply hardcode the variables you know...

Cool, knowing what you're trying to figure out is the first step.

Now that you know what you're trying to figure out, which part of that are you stuck on?

What exactly are you trying to figure out? How would you figure it out with a piece of paper and a pencil, without a computer?