Solution: If it takes X minutes to ride a bike all the way, then 200 (x-5) = 80 * (x-5+ 15), and x = 15.

Distance between Wang Chengjia and the library: 200 * 1.5 = 3000m.

A: It actually took Wang Cheng 30 minutes to get to the library from home, and Wang Chengjia is 3 kilometers away from the library.