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I'm not sure what the question is. If your rotors are thick enough to turn, turn them and put in new pads. While you have it apart, make sure the caliper is sliding on the 2 pins, It could be that one is sticking which would cause uneven wear, I guess. If you pull it apart you can clean it out and grease it lightly. Sorry my answer is vague but I haven't done it recently. If you are not sure what you are doing, have it done professionally.

 

Question is why rear pad will used that fast

 

It sounds like your calipers are either worn out or getting hung up. I would take it to somebody if you could.