"Time" in astronomy has many meanings. In essence, let us look at something called Sidereal Time.
On a fixed day, the difference between local and sidereal time is roughly the same. Sidereal time is the sum of the star's right ascension (constant) and its hour angle. The rising hour angle is given by the expression
$$\arccos(-\tan (p) \tan(d)),$$
where p is the latitude and d is the declination.
Since the latitude is "roughly" the same we only need to take longitude into account. This means that the time when an event happens, since it happens at the same sidereal time happens $S_2-S_1$ (S are local sidereal times) later in the second city (the difference is roughly the same since the difference between local and sidereal times is constant for this day).
Now, it happens 80 minutes later since 20 degrees to 360 is 80 min to 24h.