30-35,000Km (20,000+ miles) is pretty good going for any sprocket and the chain. The only thing that's odd about your story is that the back one has worn worst. Other than that, I'd be reasonably satisfied if got that distance out of C&S set, especially if you never cleaned it and only used spray lube.

If the chain and front sprocket are still good (and money is tight), you could feasibly just change the rear. Up to you really. All I'd say is, if the chain and front sprocket have got less than 10,000Km left in them, it would probably be less hassle to do the whole lot - at least then you'd know you had a definite 30,000Km of life in the drive train.

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