If the distance is x, the speed of car A is x/4/ hour, and the speed of car B is x/6/ hour.

The speed of traveling in the same direction is x/4+x/6.

The arrival time is: x/(x/4+x/6)= 12/5=2.4 hours. (2 hours and 24 minutes)

12. Add 12.

The workload per person per day is: (3/5)/(9x14) =1(14x15).

Assuming that X people can do 2/5 in 4 days, then (2/5)/[x/(14x15)] =1.

Solution: x=2 1

Therefore, it is necessary to increase 12 people.