其它
Grace was going to meet her friends at the bus station. Right now, it is $8:40$. Grace arrived at the bus station half an hour ago. The trip from her home to the station was $1$ hour long. What time did Grace depart toward the bus station? 
Overseas Competition->Knowledge Point->Combinatorics->Time Problem
Grace arrived at the bus station at $8:10$, and the trip was $1$ hour long, so she departed at $7:10$. 