In two common species of flowers, a and b, the proportions of flowers that are blue are pa and pb , respectively. Suppose that independent random samples of 50 species-a flowers and 100 species-b flowers are selected. Let pˆa be the sample proportion of blue species-a flowers and pˆb be the sample proportion of blue species-b flowers. What is the mean of the sampling distribution of pˆa−pˆb ?.

It depends on the subject for me. some exams are easier than others so i usually go through it and answer questions i know based on my general knowledge, then double check after. going through the lesson can a lot  because the exams can be  very lesson-specific, but sometimes they are ridiculously long so a lot of people don't take the time. i did find that, when i went through the lessons, i was making 100's on my exams vs strictly using + the internet and  i make 90's. 

Its probably the possible factors tt, tt, tt,