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20 G

This is easier than it sounds. There are 10 survivors in total, including Dick the pawnbroker. Here's a list with their locations:

1: Dick (Pawnbroker) you literally can't miss this guy. On the van immediately outside the safe house.

2 & 3: Gemini & Fausto. Located in the diner, the building just above the petrol station on the map. The guy will ask you for 2 drinks, just pick them up off the bar or floor and give them him. You will need to talk to them both to get them to follow.

4, 5 & 6: Sharon, Tia and Nikki. 2 of them are found in the bowling alley (north of pawn shop.) They will ask you to find Sharon who is in the quarantine area (North, where you found the first zombrex jab.) You will need to give her some zombrex to persuade her to come with you which you will need to buy from the shop for $25k. If you need cash, just smash all the fruit machines in the casino. When you've given her this, you will have to carry her back to the bowling alley and her mates will follow.

7 & 8: Jason and Archie. The 2 guys are located in the car park of the hardware store (North west-ish on the map) You need to meet these as part of the storyline anyway because one of them has the bike handlebars. They will follow you when you trade a sword for the handlebars. -To get the sword you need to open the 'Fish 'n' hunt' shop by climbing the rooftop via the hotel window and following it round.

9 & 10: Bob and his daughter Darcie. By now you would've already have spoken to Bob on the rooftop (opposite pawn shop.) These were the last 2 I got, and I rescued them after giving Katey her zombrex (7pm) so you do have time before the military arrive. Go speak to Bob and he will send you down to Fish 'n' hunt. Go in there, talk to the girl, she will send you back to Bob, after doing this a few times, they will follow.

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Extra: Further info for survivors 9 & 10. You must beat Jedd reasonably quickly to get back to Bob in time.

If you are having difficulties with this, it may be worth saving in the safehouse at about half 6, then run to Bob and be next to him before the 7pm cutscene. Speak to him immediately after the scene and it should trigger the last survivors. You can still make quick dash back to the safehouse with very little time to spare to give Katey her Zombrex but the achievement actually unlocks the moment you have Bob and his daughter through the door. So if you've already defeated Jed and completed the game you dont have to do it again.

Credit to the Interwebs for this extra info.
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Achievement unlocked. This is the best order in my opinion. Hope this helps!