ANSWER
[tex]1.28\text{ weeks}[/tex]EXPLANATION
The rate at which they built the road is 3 km per week and they built 3.84 km of the road.
To find the time it took to make the road, divide the distance made by the rate:
[tex]\begin{gathered} rate=\frac{\text{distance}}{\text{time}} \\ \text{time}=\frac{\text{distance}}{\text{rate}} \end{gathered}[/tex]Therefore, the time taken (in weeks) is:
[tex]\begin{gathered} \text{time}=\frac{3.84}{3} \\ \text{time}=1.28\text{ weeks} \end{gathered}[/tex]That is the answer.