Xiao Li bought a new watch and checked it with the wall clock at home. He found that the watch walked three minutes slower than the wall clock a day. Later, he proofread the wall clock at home with the standard time of the radio station and found that the wall clock walked three minutes faster than the standard time of the radio station. 
Xiao Li inferred that his watch was on time.
Xiao Li's inference is correct, because the watch is three minutes slower than the wall clock, and the wall clock is three minutes faster than standard time, which shows that the watch is on time