Simplify the following text for a Grade 9 native English-speaking person.
Original: The two tribes , Koror and Ulong , then participated in the first reward\/immunity challenge , with the winner , Koror , deciding to leave their present beach to another one they had not seen .
Simplified: The two tribes , Koror and Ulong , then had their first reward\/immunity challenge , with the winner , Koror , chose to leave the beach they were at and go to another one they had not seen .