Answer:
I think he will run into his ex-girlfriend, but he won't care.
Explanation:
The given sentence is a compound sentence, as it consists of two independent clauses:
- I think he will run into his ex-girlfriend.
- He won't care.
These sentences are connected by the coordinating conjunction but. Coordinating conjunctions that connect two independent clauses are always preceded by a comma, and that's why there should be a comma between the words ex-girlfriend and but.
