At 12:00 it was correct, so after that the hours were 18 minutes shorter because it says that the watch lost 18 minutes per hour.
We'll note one hour with x.
12:00 + 42/60 x +4x=15:30
Because one hour has 60 minutes, then 30 minutes means 1/2 hour, so 15:30 means 15,5
12+42/60x+4x=15,5
After solving this ecuation you'll find x= 35/47, which means that there around 9,5 hours lost.
