Question 280084
No idea what "excellerates" means. It is not a word. BTW neither is "striaght"
Perhaps you mean "accelerates" and "straight"
We are not told if the cars start at the same time or one hour after the other.
car A
50x=300 
x=6 hrs
Car B
60x=300
x=5 hrs +2*30 mins =6 hrs
Car C
75x=300
x=4 hrs+ 2 hr=6 hrs

They all arrive at the same time.
or Car A arrives first because it didn't have to keep accelerating.