Question: go to a library or some other place where there...
Go to a library or some other place where there are a lot of books. Take any 10 books at random and note the number of pages. What are the number-average and weight-average number of pages of these books? Why is the weight-average number of pages greater than the number average?
What is the polydispersity index? Is there a situation where it (polydispersity index) would be unity?