Because when A went to the library, B had 1/5 left, which means B had 4/5 left; Then, the speed ratio of A and B is 5: 4.

Divide the whole process into 10. When Party A is halfway through (5 copies), there are only 4 copies left and 6 copies left, corresponding to 768km. Divide 768 by 6 to get 128, then multiply 128 by 10 to get the whole journey: 1280.

But this question is unrealistic. Is the accessible library 1280 km?