The first step is to add up the totals for each characteristic, and enter them in the column labelled f. The first row has already been completed for you:
Samples 1, 8, 11, and 17 each had one case of blisters, giving a total of 4 cases across all 20 samples. This total is then entered into the appropriate box in the f column.
Complete the rest of the rows in the same way, entering the totals into the text boxes provided, then click the button to check your answers.