Would appreciate any help with this question! I know that the formula to be used is ?hat = ? yi bar/ ns but I'm not sure what ns is and also if yi bar is just the sum of the volume. Thank you!

6. Foresters are interested in determining the mean timber volume per acre for 520 one acre plots (N 2 520). A 1in25 systematic sample is conducted. Using the data presented in the accompanying table, estimate [1, the average timber volume per plot and place a bound on error of estimation. Plot Volume Plot Volume (board feet) (board feet) 4 7030 279 7540 29 6720 304 6720 54 6850 329 6900 79 7210 354 7200 104 7150 379 7100 129 7370 404 6860 154 7000 429 6800 179 6930 454 7050 204 6570 479 7420 229 6910 504 7090 254 7380