cuz there are 16 terms and you are subtracting 50 from each of them, which is effectively equal to subtracting (16 x 50) from the total 
yes, that is the correct formula, but there are many ways to rearrange it. the form used in this question was (Σ(x)^2)/n - (mean)^2, and we substituted Σx/n for the mean... you can find them at the end of the syllabus, and they're also given in MF9 that you get in the exam so personally, i don't remember them cuz i'm bad at rattafication anyway
yes, that is the correct formula, but there are many ways to rearrange it. the form used in this question was (Σ(x)^2)/n - (mean)^2, and we substituted Σx/n for the mean... you can find them at the end of the syllabus, and they're also given in MF9 that you get in the exam
so personally, i don't remember them cuz i'm bad at rattafication anyway