Answer
38 mL of propyl alcohol solution are necessary to obtain 4.5 mL of propyl alcohol.
Work Step by Step
$12\%$ (v/v) $ propyl \space alcohol $ solution : $$ 12 \space mL \space propyl \space alcohol = 100 \space mL \space solution$$
$$4.5 \space mL \space propyl \space alcohol \times \frac{100 \space mL \space solution}{12 \space mL \space propyl \space alcohol} = 38 \space mL \space solution$$
