A train's timetable reads — leaves Patna 06:40, reaches Gaya 08:25, leaves Gaya 08:35, reaches Ranchi 12:05. For how long was the train actually running, not counting the halt?

Patna to Gaya runs from 06:40 to 08:25, that is 1 hour 45 minutes, and Gaya to Ranchi runs from 08:35 to 12:05, that is 3 hours 30 minutes. The two running legs add to 5 hours 15 minutes. The whole journey from 06:40 to 12:05 lasts 5 hours 25 minutes, which still includes the 10-minute halt at Gaya; taking the halt off twice leaves 5 hours 5 minutes, and adding it instead of removing it gives 5 hours 35 minutes.