Answer:
The number of students who prefer chocolate milk is 780 . 
Step-by-step explanation:
Given as :
The total number of students in the school = 1200
The percentage of students who prefer plain white milk = 35 %
Let the number of students who prefer chocolate milk = x 
Now, ∵ The percentage of students who prefer plain white milk = 35 %
∴ The percentage of students who prefer chocolate milk = 100 % - 35 % = 65%
So , As The number of  students who prefer chocolate milk = x
Or, 65 % of total number of students in school = x
So, x =  × 1200
 × 1200
or, x = 
∴  x = 780
So, the number of students who prefer chocolate milk = x = 780
And  students who prefer plain white milk = 1200 - x = 1200 - 780 = 420
Hence, The number of students who prefer chocolate milk is 780 . Answer