Answer:
The total number of students is 800.
Explanation:
Given that in an examination, 40% students passed in Maths only, 30% passed in Science only and 10% students failed in both the subjects, to determine the total number of students if 400 students passed in Science the following calculation must be performed: 
 
100 - 40 - 30 - 10 = 20 
20 + 30 = 50 
 
50 = 400 
100 = X 
100 x 400/50 = X 
800 = X 
 
Thus, the total number of students is 800.