Answer
$$12.5 \ ha$$
Work Step by Step
The area of the field is $$\frac{1}{4} \times \frac{1}{2} \ km^2=\frac{1}{8} \ km^2=\frac{10^6}{8} \ m^2 .$$
Now change it to $ha$ by dividing by $10^4$; that is
$$\frac{10^6}{8} \ m^2= \frac{10^6}{8\times 10^4} \ ha =\frac{100}{8} \ ha=12.5 \ ha.$$
