The correct answer is $95.
This is one of many confusing methods that swindling merchants have used for years. There is a simpler way to figure it out:
Let x = cost of camera
Let y = cost of case
x=y+300
y=310-x
x=310-x+300
x=305, y=5
Therefore, the case costs $5, and you should get $95 change.
Yet, alot of us don't use simple algebric equations in our daily lives, and so we trust the merchant who is about to cheapskate us to give us the correct amount of money. We really need to watch out for those swindling merchants, huh?

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So, Clawandfang, Tails_155, Mornai and lbt/cty_lover all get 50 points and 2 Hint Points.
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Score thus far:
Stitch: 10 points (10 Hint Points)
Flathead770: 0 points (10 Hint Points)
Mornai: 50 points (12 Hint Points)
Clawandfang: 185 points (14 Hint Points)
LBTFan13: 0 points (10 Hint Points)
Tails_155: 80 points (12 Hint Points)
lbt/cty_lover: 80 points (12 Hint Points)
Mumbling: 35 points (11 Hint Points)
action9000: 0 points (10 Hint Points)
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Okay, your turn, Clawandfang.
