#### Answer

16.2 % of ethanol by mass

#### Work Step by Step

Assuming 100 mL of solution:
$100 \space mL \space solution \times \frac{20.0 mL \space ethanol}{100 \space mL \space solution } = 20.0 \space mL \space ethanol$
$$20.0 \space mL \space ethanol \times \frac{0.789 \space g}{1 \space mL} = 15.8 \space g$$
$$100 \space mL \space solution \times \frac{0.977 \space g}{1 \space mL} = 97.7 \space g$$
Mass percent (ethanol) = $\frac{15.8 \space g \space ethanol}{97.7 \space g \space solution} \times 100\%= 16.2 \%$