Question 1062127

Let P = the percentage score, expressed as percentage
Let c = number of correct answers
Let n = total number of questions

    P = (c/n) * 100

[ "Percent" literally means "per 100" so  given, say, 90%, you can also write that as 90/100 or 0.90 ]

In this specific question, number of correct answers = 90% * 20 = 0.90*20 = 18

So  the score 90%  = (18/20)*100
                                                
I hope this makes sense.  Just remember that percent is the same as "per 100" and that people use it as a convenient way of scoring/measuring/quantifying things.  Also, remember that given a value expressed as a percentage (i.e. with the % sign following the number) you can simply divide that number by 100 to convert to a decimal that you can use in your calculations.