You've just arrived at the beach with your daughter and her best friend, who is coincidentally the daughter of your best friend. You put your beach bag down and tell the girls, who are eager to play, that they can swim close to the shoreline but must stay away from the rocky enclave because the water is too rough. You also warn them to be extra careful because this is a private beach, which means there are no lifeguards on duty. The girls agree and leave you behind.

 

As you are setting up, you hear the girls screaming and can tell it isn't playful. You turn around to see that they didn't listen to you and were being swept out by a rip tide. As you are swimming out, you are having trouble yourself. You know you can save both of them, but it will have to be one at a time. Your daughter is a relatively strong swimmer, but her friend is struggling to keep her head above water. 

You think there is a 50% chance that your daughter could wait for you to return, but know her friend will drown if you leave her. What do you do?