Mathematics and Physics. How many students failed in the class ?

The correct answer is (c), 35.

In the class of 100 students, 25 failed Mathematics and 30 failed Physics. However, 5 students failed both Mathematics and Physics, so they are counted twice in the total number of failures. Therefore, the total number of students who failed in the class is 25 + 30 – 5 = 35.

Option (a), 25, is incorrect because it only includes the number of students who failed Mathematics. Option (b), 30, is incorrect because it only includes the number of students who failed Physics. Option (d), 45, is incorrect because it includes the number of students who failed both Mathematics and Physics twice.