I think your math is off but it might be my reading comprehension
You can do this several ways. I'll do the math assuming a total of 125 questions with 85 questions answered correctly and 40 questions answered incorrectly.
If there are 125 questions, dividing 100 by 125 will give you a value of .8% per question. Take the number you got right and multiply by .8 to get your percentage correct. So 85 x .8 = 68%
Or, you can multiply the number you got wrong by .8 and subtract from 100 (not 125 since we've already converted the value of each question into a percentage of 100). So 40 x .8 = 32% And 100 - 32 = 68%
The method your friend gave will also work since it's the number correct divided by 125 and then multiplied by 100 which gives you the percentage correct. 85/125 = .68 And .68 x 100 = 68%
Every method will give you the same percentage it's just slightly different ways of doing the math based on what makes sense to you.