To solve this problem we need to apply the average speed formula, which is shown below:
[tex]\text{speed = }\frac{dis\tan ce}{time}[/tex]We need to find the time, therefore we will arrange the equation in order to isolate the time on the left side as shown below:
[tex]\begin{gathered} \text{time}\cdot\text{speed = distance} \\ \text{time = }\frac{dis\tan ce}{\text{speed}} \end{gathered}[/tex]Applying the data from the problem:
[tex]\begin{gathered} \text{time = }\frac{1920}{640} \\ \text{time = 3 h} \end{gathered}[/tex]