If your questions are all going to have different 'weight' values, one way to do this (even though its not the most scalable) is to create an [Points awarded] field for each of the questions.
Example:
[Q1] What color are banana's?
Multiple Choice Options:
A - Red
B - Yellow
C - Blue
[Q1 - Value]
(Make this a formula numeric field, with a similar equation)
If([Q1]="B - Yellow", 5, 0)
That will award 5 points for the right answer, and 0 for anything else.
Then you can take all your [Q - Value] fields and add them up, and divide by the total points possible.
[Score]
Sum([Q1 - Value], [Q2 - Value], ..... [Q20 - Value]) / 100
(Set the properties to display as a percent)
There is another way you can do it with a few tables and relationships, but if you are just doing this once, this might be your best option.