Since there are 24 hours in a day and 360 degrees in a circle, the sun moves across the sky at a rate of 15 degrees per hour.
To cover a distance of 1° (i.e., per longitude), the sun will take 4 minutes. Thus, the time difference per longitude is 4 minutes.
Now, between A and B, the number of longitudes is
Thus, the time to cover 45 longitudes is
Now, since B (at 90W) is to the west of A (at 45W), time-wise, A is ahead of B, by 3 hours. So, if at A, the local time is 10 pm, the local time at B is 7 pm.