There is 20.7 mL of solution remaining in the bottle. Since you start off with 56.6mL and then use 3.2mL of the solution and then pour half of it into a beaker and then pour 6 mL of the solution into the test tube. All you have to do is just make an equation.
56.6-3.2=53.4
53.4/2=26.7
26.7-6=20.7
