Pearson's measure of skewness -
Step 1: Subtract the median from the mean: 70.5 – 80 = -9.5.
Step 2: Divide by the standard deviation: -28.5 / 19.33 = -1.47. Caution: Pearson's first coefficient of skewness uses the mode. 
Therefore, if the mode is made up of too few pieces of data it won't be a stable measure of central tendency.
Mode - Mean/SD
Mean - Mode/SD
SD/Mode - mean
Mean - Mode/SD