Customer: "I see that the amount I owe has been reduced from $90.00 to $75.00." Employee: "Yes, the discount has saved you _____
2 answers:
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I'm thinking this is what the problem looks like: 
. The first thing to do is to move the 
over to the other side because it has a common denominator with the other side. Doing that and at the same time combining them over their common denominator looks like this: 
. The best way to solve for x now is to cross-multiply to get 3(4-x)=-4(x-4). Distributing through the parenthesis is 12 - 3x = -4x + 16. Solving for x gives us x = 4. Of course when we sub a 4 back in for x we get real problems, don't we? Dividing by zero breaks every rule in math that there ever was! So, yes, the solution is extraneous.