Question 971208
I suggest to draw the Venn diagram first because it will make you more make sense. 
ok.. so,
18 studying biology; 9 biology and chemistry; 7biology and physic.
18 - (9+7) = 2 (2 is the student who only study bio)

30 chemistry; 9 biology and chemistry; 11 studying physics and chemistry
30 - (11+9) = 10 (only chemistry)

25 physics; 7 biology and physics; 11 studying physics and chemistry
25 - (7+11) = 7 (only physic)

add all numbers and subtract from 50 because it said 50 students in a class
50 - (2+10+7+9+7+11) = 4
 
the answer is 4 students are studying all three subs......