If you get stuck on a particular mission in the game's Challenges mode spend some Duct Tape. You'll forfeit the reward, but at least you will move on.
The world comes to an end; run like hell, collect duct tape, and buy goodies for your survival camp. Once the streets are deserted, everything is covered in lava, and you are alone. Yet, you hear the call… and even in the middle of the Collapse, you know it is time to run, to survive!