What is Fair? What is Unfair?

Not everything in life is fair. What is fair and what is unfair? How do you define fair? Not every effort you put in will be rewarded. You put in 100% effort but receive no reward. Is this Fair? You put in 50% effort but with 100% reward. Is this also fair? How about no effort but was given 80% reward. Is this fair? How to define? I oso dunno.