Raghu Pasupathy

Since the time until the last ant exits the stick is requested, instead of the ants colliding and reversing direction, we can assume that they simply walk past each other. It is easily seen that this altered experiment will not change the time taken by the last ant to exit the system. So, any arrangement where at least one ant starts at one end of the stick and facing the other end will maximize the time taken for all ants to exit the stick. In such a case, the total time taken will be 100/1 = 100 seconds.