This is called Zeno's paradox, and it is a logic problem built on a faulty logic, in which Achilles can never pass the tortoise, he can only get closer and closer to it. He cannot even get even with it.
In reality, Achilles can easily pass the tortoise. I think that some form of calculus can explain mathematically how the tortoise can be passed, but the math that explains how Achilles can pass the tortoise mathematically had not been invented.