Mean is calculated by adding the numbers, then divide by the count of numbers. The count is 3 in this case. 15 + 10 + 15 = 40, so mean = 40/3 = 13.3333.....
11.82 To find the mean of something, add all the figures and then divide by the number of figures. You would save youself time by just doing this in a calculator instead of posting it here.