Rescue Sinthia

Save Sinthia is a side quest in Fallout.

Quick walkthrough
If the Vault Dweller spends a night in the Crash House Hotel in Junktown, they'll wake up to find the hooker Sinthia held hostage by a random raider in the room adjacent to theirs. Marcelles will beg the Vault Dweller to defuse the situation.

Detailed walkthrough
There are three ways to solve this quest:

Peaceful solution
As soon as the raider can see the Vault Dweller, he will start dialogue. When talking with the raider, the Vault Dweller ought to assure him that no one's going to hurt him, in order to have him believe that the Vault Dweller is his friend and not an enemy. If their Speech skill is high enough, the Vault Dweller can convince him to leave without a fight, but if it isn't high enough they may need to pay him either 100 or 200 bottle caps, depending on what they say and when.

Violent method
Another solution is to begin talking with the crazed raider in a calming manner. If he says he needs to think about it, the Vault Dweller should walk in on him. He will attack them instead of Sinthia, allowing the Vault Dweller to kill him and save the prostitute.

Sneaky way
Again, like the option above, the Vault Dweller should talk to him in a calming manner. When the raider says he needs to think about it, the Vault Dweller should activate the Sneak skill. If it's high enough, they'll be able to sneak behind him without him reacting. Shooting him will knock him unconscious. At first, Sinthia will think he's dead, but the game confirms he's not.

Rewards

 * 1000 or 400 XP.
 * A free night at the Crash House.
 * Sinthia's "services".