Question 548346
The short answer is that you'd better shoot for answering 27 of the 30 questions correctly for a 90% in the big test.
If the final exam has 30 questions, all with the same point value and no partial credit is given for answers, then 27 correctly answered questions would give you a 90%
{{{100*27/90=90}}}
and 26 correctly answered questions would give you 87%.
{{{100*26/90=86.667}}}
More precise calculations could be done if we knew other details, but that probably would not make a practical difference.
The problem is that there are details on how grades are calculated, and the devil is in the details.
If items accounting for 65% of the grade have already been completed and graded, with a current grade of 90%, and only a final exam worth 35% of the grade remains, a 90% in that final exam would mean a 90% final grade.
Depending on how the grades are rounded, a slightly lesser grade may do. For example, if a 90.000% on 65% of the items was to be averaged with an 89.000% on a final exam worth 35% of the total grade, I would calculate the final grade as
{{{0.65*90+0.35*89=89.65}}} and would round it up to 90%.
But maybe they round up any fraction. The points suggest that. If the items graded so far added to a maximum possible of 1076 points and you earned 962 of those possible points, I would calculate your grade as
{{{100*962/1076=89.405}}}
Maybe your current grade was calculated like that, and anything with a decimal part is always rounded up to the next integer. That gives you a tiny advantage, so tiny that it's probably not worth worrying about.
Each question in the 35% exam cannot be worth 1 point of the same kind of the  962 points already attained, because 30 points make only a small percentage (2.7%) when pooled with the 1076 points already attempted.
Confusing? I spent 5 years trying to predict my youngest son's final grades, form all the grades information the school had online, but I could only come close. I bet the teachers would just input grades, a computer program would calculate final grades in a way no one but the programmer could understand, and then the teacher would apply a curve or some other fudge factor to come up with the grade we were given.