Xiao Ming's time from home to home = time spent walking from home to the library+time spent in the library for 3 hours (time difference of library clock)+time spent going home from the library.
Because of the same speed, the time from home to the library = the time from the library = 0.5 hours.
So the time of Xiao Ming's big alarm clock is 0.5 hours later than the exact time (clock time), and the exact time for Xiao Ming to get home is 1 1: 30.