The sentence with an unclear pronoun is:
"Jamal and his cousin left at the same time, but she reached the park before he did."
In this sentence, "she" could refer to either Jamal or his cousin, making it unclear. To correct it, you could specify who "she" is by using a name, for example: "Jamal and his cousin left at the same time, but Jamal's cousin reached the park before he did."