The first ten composite numbers are 4, 6, 8, 9, 10, 12, 14, 15, 16 and 18. Now, if we add the 10 composite numbers, we will have a new no. as 112. Mean is nothing but the sum of observations divided by the no. of observations.
Now as I said above, if we count the number of observations, then it is ten.
Now divide 112 by 10 and you will get the mean.