As 2 students did not opt for any class,

Number of students who opted for at least one class = 50-2 = 48.

let a be no of students who opted for only math.
let c be no of students who opted for only science.
let b be no of students who opted for both math and science.

as 10 did not opt for math, c=10.
as 15 did not opt for science, a=15.

Now, a+b+c = 48, => b = 48-(10+15) = 23.

Answer =C.

Hope this helps!

I got D by making a Table.

Something like this

____________Math____________No Math ________Total
Science_____27_____________13_______________40

No Science____8_____________2 (given)__________10(given)

Total________35_____________15(given)_______________50


Sorry misread the question the first time. The Answer should be D

total = 50
neither = 2
no_math = 10 -> math=50-10=40 (including math_only and math_and_science)
no_science = 15 -> science = 50 - 15 = 35 (including science_only and math_and_science)

Use the formula:

total = math + science - both + neither
50 = 40 + 35 - both + 2

both = 27