Dear aspirant,

Relative grading is when you are graded relatively to your peers . So, after every semester your total marks are plotted as a curve.

The median (the marks that most people in your class obtained) usually gets a B grade in the course and people above median get the higher grades.

The distribution of grades is dependent on the shape of the grades' curve and usually 0.6*median and below gets a fail grade.

So, in this system of grading your performance is assessed in relation with your peers and hence, the difficulty of the paper won't matter much.

This an example of how it happens and some colleges have different parameters for grading and deciding what the failing grade is. Hope this helps.