Notice that by the definition of dilution factor (volume of stock / total volume), the numbers in the first two colums are consistent with the numbers in the third column.

Example : (row 2) DF = 2.5/(2.5 + 0.5) = 2.5/3 = 1/1.2
In this manner you can calculate the DF for other rows.

The first row tells you the concentration of sunscreen in the stock (.0898 g/mL). Knowing how much sunscreen there is in 1 mL, you can then find out how much there will be in 2.5 mL, 2 mL, 1.5 mL, etc. (the volumes of the different stock solutions). Dividing these weights by the total volume gives you the concentration for each case.