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The Reilly's Rangers achievement for Fallout 3 is worth 20 G

I recommend having a fission battery in your pack before starting your quest so you won't have to backtrack later. If you don't have one you can get one during the quest so don't worry.

You will find Reilly passed out in the chop shop in the Underground(which is located in the museum of History). Try to awake her and then talk to the Ghoul Doctor and he will give her an injection to wake her up. When she wakes she will tell you the story of what happened and also tell you that her rangers are bogged down on the roof of the Statesman Hotel Restaurant. Also she tells you have a ranger who fell by the name of Theo. Theo carried the ammo for the rangers so you are tasked to find it. I found him on some stairs in the hotel while trying to complete the quest so he's not to hard to miss. If you enter the front door of the hotel you will find you can't go up because the elevator is shut down. There is another way to get to the upper floors though. If you are outside and looking at the front door of the hotel go right and then head behind the hotel. After about 50 yards you will see a hospital on your right enter it and head to the second floor (beware or mines and traps in this area and in the hotel itself). From the upper floors of the hospital there is a scaffold that goes across to the hotel. Take it and then work your way to the roof. There will be tons of mutants so it can be quite frustrating. When you get to the roof talk to butch. He can repair the elevator if you have a fission battery. If you don't have one, no worries there is one on the floor below in a closet. Give Butch the fission battery and then take the elevator down. You will have to fight a few more mutants when you get to the bottom floor. Exit the building and you will have to go to the Ranger Compound. When you get there talk to Reilly.