Well, It would be hard to write out the formula and everything here so I wrote it out for you! Work shown!
B is the correct answer.
8 x 35 = 280
Well to find the mean you need to add up the total number of people and then divide by the total number of days meaning if there is a total of 105 people, you would divide that by 5 and get 21
13 + 23 + 24 + 31 + 14 = 105
105 / 5 = 21