If marks in Eng = 12/25 , Maths = 40/50 , Science = 15/25 then Marks obtained/Total maks = 67/100 = 100%
If equivalent fractions are taken for all subjects to out of 100 then, Eng = 48/100 , Maths = 80/100 , Science = 60/100 , Marks obtained/ Total Marks = 188/300 = 62.66% .
The first gives math twice as much weight as the other two while the second gives all three subjects the same weight To get the same result, you'd have to give 200 points total to math (or count the 80/100 twice)