Question

After performing a dilution calculation, you determine you need 25.0 milliliters of an aqueous stock solution to make 100.0 milliliters of a new solution. How would this be prepared?

Answer 1

100.0-25.0=75.0 millimeters

Answer 2

"Add 25.0 milliliters of the stock solution to a volumetric flask, then add water to a total volume of 100.0 milliliters."

Since you want 100 milliliters in total you add 25 ml of the given and fill the flask up with water until 100 ml is reached.