Formula to calculate it is simple:
[tex]v=\frac{s}{t}[/tex]
where v is the speed, s-road, t-time
Speed is equal to the ratio of road to czau estimated driving:
[tex]65=\frac{390}{t}|*t[/tex]
[tex]65t=390|:65[/tex]
[tex]t=\frac{390}{65}[/tex]
[tex]t=6[/tex]
The car was for 6 hours.