Pipes A and B can fill a tank in 16 hours and 24 hours, respectively, and C is an emptying pipe, which alone can empty a full tank in x hours. All the pipes were opened together at 9:30 a.m., but C was closed at 12:30 p. m. If the tank gets filled at 7:30 p.m. on the same day, then what is the value of x?
1
80
2
72
3
65
4
60