The Math Percentage Average Problem is as follows :
Question
I scored 80 marks in my first internal maths paper. By how much percent should I improve my marks in the second internal in order to get a average of 90 in maths ?
Aptitude Problems Solved
Solution
There are two internals. First internal I got 80 marks. Say my marks in the 2nd internal are ‘x’.
Given that I want 90 as my average, then
90 = (80 +x)/2
Solving for x, you will get x = 100.
So I need to increase my marks by 20.
Since 20 is 25% of 80 marks, the answer is 25% 
If you have a doubt, you can post it here and Ill answer it 😉