Full marks is 75.
Explanation:
Let's set up a proportion to better understand the problem
80% 100%
----- = ----
60 ?
Since we want to know what is FULL, we know we have to find 100%. Now, to get from 80 to 100, we multiply by 1.25 (100/80 = 1.25. After we know that, all we have to do is do the same to 60. So we multiply 60 by 1.25 as well, to get 75. Therefore, full marks is 75.