A researcher wishes to​ estimate, with ​90% ​confidence, the population proportion of adults who are confident with their​ country's banking system. His estimate must be accurate within 3​% of the population proportion. ​(a) No preliminary estimate is available. Find the minimum sample size needed. ​(b) Find the minimum sample size​ needed, using a prior study that found that 26​% of the respondents said they are confident with their​ country's banking system. ​(c) Compare the results from parts ​(a) and ​(b).