Dude this question 100% incorrect (If you have posted here correctly).

It initially states that the ratio of Math to Non-Math is 2:3 i.e Out of every five students there are 2 math students = 40% strength is of Math students.

Now if we add two more Math student in the class the new ratio would be 1:2 i.e. out of every 3 student there are 1 math student. It means new math student's strength = 33.33%

How can be the percentage of math student would reduced on adding students in math section.

So, this question is 100% incorrect. BTW From which source did you pick this question???

I agree that it's a flawed question. Not only does your equation yield a negative number, but also if the answer really is 28, the number of math majors would not be a whole number.

Ask for a refund?

Rey

It is correct:

Look at it this way:

2 to 5 = 7
4 & 10 = 14
6 & 15 = 21
8 & 20 equal 28 --> add 2 and you have 10 & 20, or 1 to 2.
10 & 24 equal 35

Source is the official study material sent out by GMAT, test code 28.

If the initial ratio of math and science is 2:5, then this is correct. Please check the question once again

In a certain calculus class, the ratio of the number of mathematics majors to the number of students who are not mathematics majors is 2 to 5. If 2 more mathematics majors were to enter the class, the ratio would be 1 to 2. How many students are in the class?

a- 10
b- 12
c- 21
d- 28
e- 35

Answer D

majors/no majors 2/5 become 1/2
homonyms: 4/10 5/10
multiply by 2: 8/20 10/20
Difference between numerators is 2
Therefore 8+20=28 Answer is D.