Diamantino put three liters of water and one liter of soft drink in a container. The soft drink is composed of $20 \%$ orange juice and $80 \%$ 
water. After mixing everything, what percentage of the final volume represents the orange juice?
(a) $5 \%$
(b) $7 \%$
(c) $8 \%$
(d) $20 \%$
(e) $60 \%$
The correct option is (a).

The refreshment is composed of $20 \%$ of a liter, that is, 0.2 liters of juice and $80 \%$ of a liter, that is, 0.8 liters of water. Therefore, the final mixture has 0.2 liters of juice and $3+0.8=3.8$ liters of water. The percentage of juice in relation to the volume of the mixture is then,

$$
\frac{\text { volume of juice }}{\text { total volume }}=\frac{0.2}{4}=\frac{2}{40}=\frac{5}{100}=5 \%
$$