Answer
$$16.7 \ \% .$$
Work Step by Step
Since $1~gal = 4~qt$, then the total capacity of the radiator is $1~gal +5~gal=6~gal$. So the percent of antifreeze in the radiator will be given by the ratio of antifreeze and the total capacity. That is, it is given by $$ \frac{1}{6}\times 100 = 16.7 \ \% .$$
