The mean is of a dataset is calculated by the sum of all data and then divide it between the size of the data, the number of scores.
Let's start adding all scores:
Now the second step, count the number of points or the size of the dataset. There is 12 scores in total. Next, let's divide the sum 285 between 12:
Finally, the question ask to round to the nearest tenth, hence the answer is 23.7