What should I do for this maths question?

I know I have posted this question before however I find that the answers are wrong.. Anyway, the question is Achilles speed is 20km/h and the tortoise speed is 2km/h. The tortoise has a 100m head start. When they started running, when will Achilles ever catch up with the tortoise? Note that the tortoise is also running. Many people said that the answer was 20 sec. It is not that I do not want to trust them. It is just that this is for an important maths project.