In an attempt to be environmentally friendly, the city council decides that there must be at least 18 trees on each block in the city. A sample of 20 blocks was randomly surveyed, with these results (number of trees per block): 24, 17, 7, 20, 28, 27, 10, 21, 30, 8, 30, 27, 22, 27, 27, 13, 27, 20, 24, 17. What is the sample mean number of trees per block (x¯)?

17.7
21.3
24
there is insufficient data to answer this question