Question

In an examination of a certain class, at least 70% of the students failed in Physics, at least 72% failed in Chemistry, at least 80% failed in Mathematics and at least 85% failed in English. How many at lest must have failed in all the four subjects ?

Answer 1

Let's start with Physics and Chemistry.

at most 30% passed physics
at most 28% passed chemistry
so, at most 58% passed either of the two subjects.
That is, the % who failed both is at least 42%

That is 100-(30+28)%

Now extend that to the other subjects.