Why do we use average of two numbers during calculation of percentile? Let me give you example:  "908". I see how we arrive to such number.(10*0.4=4, then we take the fourth film from the bottom, it's "Jaws". The next film after "Jaws" is "Titanic". Then we find their average, it's (907+909)/2=908). The problem is, it just doesn't make any sense. As far as I understand, "Identify the 40th percentile" means "Find how much grossed film that grossed more than the bottom 40% of films". And there is no need to calculate imaginary film that grossed 908 millions, there is already film that grossed more than bottom 40% of films, it's "Titanic". And it grossed 909 millions

It seems like I figured it out. "Find how much grossed film that grossed more than the bottom 40% of films" is only half of problem. The full problem must sound as "Find how much grossed film that grossed more than the bottom 40% of films AND grossed less than top 60% of films". "Titanic" can't fullfill the second part because it grossed less than top 50% of movies. On the other hand, a film between "Jaws" and "Titanic" would be able to fullfill both parts. Also, we find such film in right the middle between "Jaws" and "Titanic" in order to avoid bias.