Continue the pattern 0,1,3,7,15,31, blank

Ineed to learn how to solve this problem

Asap! i'm not good math and i'm lost.

Identify all the cases where non-probability sampling is used when attempting to study the characteristics of a large city. studies on every subject in the population divide a population in groups and obtain a sample studies with a limited amount of time demonstrate existing traits in a population studies with the nth person selected