Transportation Time Problem for Solver Add-In

I have a fairly straightforward problem but have limited experience with Solver and not enough time to research and learn myself. Hoping someone can help. This is the problem: I have 700 patients that need to be evacuated from a city. I need to calculate how many ambulances it will take to evacuate all 700 within 96 hours. Here is the problem, 420 of them are going to one city and 280 are going to another city. I have the total, round trip transportation time for each destination. A simple solution is to simply calculate how many ambulances it would take to do the job to each destination within the 96 hours however there would be overlap and so I know that there is probably a more efficient solution. Any help would be greatly appreciated.

Interesting homework. Had a go at it testing the Simplex-LP but had to do two runs to get the most optimal solution.

First run I minimized number of ambulances
LP_1.jpg

then I changed the model a bit to minimize number of runs
LP_2.jpg

As solver complained and not finding a solution if I tried to change both parameters in one go I also tested the evolutionary solver but solution was not as good even if it was close.
evo_sol.jpg

Well it could be I did not find the right setup for this model. Perhaps another forum member may do better?

Alf