For a sample of 15 students at an elementary-school snack bar, the following sales amounts arranged in ascending order of magnitude are observed: $0.10, 0.10, 0.25, 0.25, 0.25, 0.35, 0.40, 0.53, 0.90, 1.25, 1.35, 2.45, 2.71, 3.09, 4.10. Determine the (a) mean, (b) median, (c) mode, and (d) skewness for these sales amounts.