Hey there,
The percentage equivalent of a score is found by dividing that score by the maximum possible score and then multiplying by 100.
Introduction to Percentages[
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For example, "23 out of 25" => 23 / 25 * 100% = 92%
To do this in C# you will indeed have to create more variables - though it's very straightforward provided you understand the math above :)
Best of luck!