Adding times to times

I posted earlier but it is not appearing anywhere so reposting.

I am trying to get the formula so that I can add the travel time based on miles traveled to a departing time to get the arrival time.

For instance,

Say I leave New York at 13:00 hours [A]
I am traveling at 30 miles per hour [B] (the mph would be user input so would be variable)
I need to travel 90 miles [C] (this would be user input so would be variable)
What time will I arrive? [D] (in this example it would be 16:00 hours)

Normally, it would be something like A+(C/B)=D but I get answers that are way off. I am unable to add the traveling time (180 minutes in the above example) to the departing time [A] to arrive at a correct answer for [D].

Thanks!