2nd Question needs more elaboration

Hi Alaa, Hope you are doing well.

The question itself says "You go to Silicon Valley and visit 200 start-ups in a random manner"

Visiting 200 start-ups in a silicon valley alone is a representative sample, but visiting 200 start-ups in a random manner cannot be considered as representative.

Picking from NYU canteen is considerably representative, coz they are considering only students from canteen as a whole. 

Have a happy learning!

Yes, you pick random start-ups in Silicon Valley, but you want the average valuation in the USA --> so by only selecting Silicon Valley companies, companies outside this region don't have the same chance of being picked, meaning that apart from not being representative, it's also not random?