Answer:
20.7 mL
Step-by-step explanation:
There is 56.6 mL of solution in the bottle. 3.2 mL is used up.
This leaves us with 53.4 mL (56.6 - 3.2). Then, half of this amount is used up.
This leaves us with 26.7 mL (53.4 ÷ 2).
Then, 6 mL of this is poured out. This leaves us with 20.7 mL (26.7 - 6).
