How much glycerol (is liquid supplied at 100%) would you need to make 200 mL of 20% v/v (volume/volume) glycerol solution?

40mL of glycerol are needed to make a 20% v/v solution

Explanation:

This problem can be solved with a simple rule of three:

20% v/v is a sort of concentration. In this case, 20 mL of solute are contained in 100 mL of solution.

Therefore, in 100 mL of solution you have 20 mL of solvent (glycerol)

In 200 mL, you would have, (200.20) / 100 = 40 mL